-__      _J 


I 

» 


MINE  SAMPLING 

:    '  ...  AND     ,':':      '  ; 

VALUING 


A  Discussion  of  the  Methods  Used  in  Sampling  and 
Valuing  Ore  Deposits  with  Especial  Refer- 
ence to  the  Work  of  Valuation  by 
the  Independent  Engineer 


BY 

C.   S.   HERZIG 


WITH  A  CHAPTER 

ON 

SAMPLING  PLACER  DEPOSITS 

BY 

CHESTER  WELLS  PURINGTON 


PUBLISHED  BY  THE 
MINING  AND  SCIENTIFIC  PRESS,  San  Francisco 

AND 

THE  MINING  MAGAZINE,  London 

(Printed  in  U.  S.  A.) 

1914 


COPYRIGHT  1914 

BY 
DEWEY  PUBLISHING  COMPANY 


This    Book     is     Dedicated     to 

T.     A.    RICKARD 
"He   Blazed   the   Trail" 


291134 


CONTENTS. 


INTRODUCTION     7 

PART  I. 

MINE  SAMPLING. 
CHAPTER  I. 

PRINCIPLES    UNDERLYING    MINE    SAMPLING    AND    VALUING.     Definition    of 

ore.     Sampling  for  valuation    11 

CHAPTER  II. 

PICK     ANALYSIS.      Objects.      Work     preliminary     to     sampling.      Optical 

inspection    16 

CHAPTER   III. 
IMPLEMENTS  USED  IN  SAMPLING 19 

CHAPTER  IV. 

GENERAL  METHODS  OF  SAMPLING.  Preparation  of  exposure.  Quantity  per 
foot  sampled.  Interval  sampling.  Fractional  sampling.  Sampling 
rises  and  winzes.  Sampling  cross-cuts.  Measurement  of  width 
sampled 28 

CHAPTER  V. 
PRECAUTIONS  NECESSARY  IN   SAMPLING 45 

CHAPTER  VI. 
GEOLOGICAL  FACTORS  IN  SAMPLING  AND  VALUATION 50 

CHAPTER  VII. 
CHURN  DRILLING  AS  APPLIED  IN  SAMPLING 57 

CHAPTER  VIII. 

HANDLING  OF  SAMPLES.     Numbering  the  samples.     Description  of  sample. 

Sacking   the    sample.     Tying   and    sealing    64 

CHAPTER   IX. 

PREPARATION  OF  SAMPLES  FOR  ASSAY.     Division  of  sample.     Fineness  of 

crushing.     Preparation   of  the   pulp.     Final   sample 69 

CHAPTER  X. 
ASSAYING  OF  SAMPLES   .  77 


6  CONTENTS 

PART  II. 

MINE  VALUING. 
CHAPTER  XI. 

ESTIMATION  OF  ORE.  Calculation  of  averages.  Check  sampling.  Erratic 
high  assays.  Assay  plans.  Calculation  of  tonnage.  Inaccessible  ore. 
Irregularly  spaced  drill  holes  83 

CHAPTER  XII. 
ORE  IN   SIGHT.     Definitions.     Ore  Reserve  plans 103 

CHAPTER  XIII. 

CALCULATION    OF    PROFITS.     Treatment    problems.     Working    costs.     Base 

metal  prices  Ill 

CHAPTER  XIV. 
AMORTIZATION  OF  CAPITAL  116 

CHAPTER  XV. 
WRITING  REPORTS    120 

CHAPTER  XVI. 
SALTING  125 

CHAPTER  XVII. 
PROSPECTS.     New   discoveries.     Re-opened  old  mines    129 

CHAPTER  XVIII. 

A  FEW  SPECIAL  CASES.  Miners  as  samplers.  Checking  surveys.  Dress- 
ing mines  for  sale.  Misrepresentation  of  facts 132 

CHAPTER  XIX. 
SPECIFIC  GRAVITY  137 

CHAPTER  XX. 
THE    SAMPLING    OF    PLACER    DEPOSITS.     By    Chester    Wells    Purington. 

PRELIMINARY.  General  classes  of  deposits.  Characteristics  of  the  pay 
layer.  Origin  of  auriferous  plains.  Distribution  of  pay  channels  in 
the  plains.  Preliminary  remarks  to  sampling.  Conditions  affecting 
value  of  property.  Instruments  and  equipment.  Rocker  and  clean-up 
boxes.  Hand  pumps.  Prospecting  by  shaft-sinking.  Type  of  drill. 
Drilling  with  power  drills.  Calculation  of  value.  Drilling  with  hand 
drills  ....  140 


INTRODUCTION. 


In  the  past  a  great  deal  has  been  written  on  the  subject  of 
mine  sampling  and  valuation,  but  for  the  most  part  the  literature 
is  fragmentary,  and  I  believe  that  scarcely  anywhere  is  there  any  refer- 
ence made  to  its  elementary  side.  Somehow  or  other,  engineers 
pick  up  a  knowledge  of  this  part  of  the  business,  although  I  do  not 
think  that  the  methods  of  mine  sampling  are  taught  in  schools  of 
mines,  nor  do  the  text-books  on  mining  deal  with  it  in  detail. 

A  few  years  ago  T.  A.  Rickard,  who  at  that  time  was  editor  of 
the  Engineering  and  Mining  Journal  of  New  York,  wrote  a  series 
of  articles  that  were  followed  by  contributions  from  various  en- 
gineers. These  articles  and  the  ensuing  discussion  were  eventually 
published  in  book  form  with  the  title  of  'The  Sampling  and  Estima- 
tion of  Ore  in  a  Mine/  Aside  from  this,  no  attempt  at  a  compre- 
hensive work  on  the  subject  exists,  and  for  that  reason  Mr.  Rickard's 
book  has  since  then  served  as  a  sort  of  text-book  to  the  younger 
engineers  desiring  information  on  this  phase  of  mining  engineering. 
The  discussion  contained  therein  was  a  most  valuable  one  at  the 
time,  but  on  the  whole  it  lacks  that  continuity  of  thought  which  the 
subject  deserves. 

We  owe,  too,  a  great  deal  to  H.  C.  Hoover,  who,  in  his  book 
'Principles  of  Mining,'  has  contributed  a  valuable  discussion  on  mine 
valuation.  It  is  a  source  of  wonder  to  me  that  this  matter  has  not 
been  seriously  taken  in  hand  heretofore,  because  it  must  be  remem- 
bered that  the  whole  success  of  mining  operations  is  often  absolutely 
dependent  on  accurate  mine  sampling.  I  therefore  trust  that  the 
following  pages  may  serve  as  a  guide  to  the  young  engineer  and  may 
not  be  without  value  to  the  more  experienced  and  older  men. 

I  am  indebted  to  Messrs.  Frank  H.  Probert  and  Lloyd  T. 
Bnell  for  information  regarding  churn  drilling,  as  also  to  E.  N. 
Skinner;  to  H.  S.  Munroe  for  his  views  on  the  gravimetric  method 
of  calculating  averages  and  to  others  who  have  been  good  enough 
to  offer  suggestions  of  various  kinds. 

C.  S.  HERZIG. 
London,  December,  1913. 


PART  I. 
MINE  SAMPLING. 


CHAPTER  I. 

PRINCIPLES    UNDERLYING    MINE    SAMPLING    AND 

VALUING. 

Briefly  stated,  the  object  of  mine  sampling  is  to  determine  the 
grade  of  the  ore  exposed  in  a  mine,  and  thereby  lead  to  an  esti- 
mation of  the  assay-value  and  tonnage  of  the  orebody.  Mine 
valuation,  on  the  other  hand,  is  a  broader  subject,  comprising 
many  ramifications,  and  requires  technical  skill  and  experience,  as 
well  as  business  ability. 

The  principles  underlying  the  sampling,  of  mineral  deposits 
are  the  same  whatsoever  the  kind  of  mineral,  be  it  gold,  silver, 
copper,  lead,  tin,  iron,  or  any  other.  Variations  in  the  methods 
employed  depend  on  the  intrinsic  or  commercial  value  of  the 
mineral,  its  regularity  of  distribution  in  the  gangue  and  the 
idiosyncrasies  of  the  engineer.  The  object  to  be  attained 'is  to 
estimate  as  nearly  as  possible  the  economic  value  of  the  ore, 
keeping  in  view  the  cost  of  the  examination. 

The  estimation  of  the  tonnage  and  assay-value  of  an  ore  de- 
posit is  an  attempt  to  put  into  exact  figures  the  result  of  inexact 
work.  With  all  the  care  possible  in  the  sampling  operations,  the 
samples  taken  represent  only  the  outer  skin  of  a  block  of  ground 
and  the  next  face  may  be  100  ft.  distant.  Given  an  orebody  ex- 
posed by  certain  workings,  the  problem  is  to  determine  the 
numbers  of  tons  of  material  that  can  be  mined  from  it  and  what 
will  be  its  mineral  or  metallic  contents  when  so  mined.  The 
adjustment  to  meet  the  condition  of  profit  as  dependent  on  metal- 
lurgical or  economic  conditions  is  a  subsequent  operation  that 
calls  for  wide  experience  and  especial  talent  on  the  part  of  the 
engineer.  Not  all  engineers  of  experience  are  good  mine  valuers, 
any  more  than  all  experts  with  oils  are  painters. 

A  successful  mine  valuer  requires  an  unusual  combination  of 
qualities.  He  needs  to  be  experienced,  observant,  analytical,  con- 
structive, bold  yet  cautious,  far-seeing  and  have  the  commercial 
instinct  strongly  developed.  The  commercial  instinct  is  the  one 
quality  the  engineer  most  rarely  possesses  and  the  lack  of  it  is 
the  cause  of  many  disasters.  It  must  be  borne  in  mind  that  under- 
valuation is  just  as  bad  engineering  as  overvaluation.  The  former 
is  largely  due  to  timidity  or  excessive  caution  and  the  latter  to 
excessive  optimism.  The  engineer's  work  should  be  constructive. 
The  over-cautious  mine  valuer  is  destructive ;  he  not  only  destroys 
business,  but  often  unjustly  destroys  the  fruits  of  other  men's 
labor.  In  doing  so  he  violates  his  function  as  an  engineer. 

Psychology  plays  no  unimportant  part.  A  man's  first  impres- 
sion is  apt  to  affect  his  final  judgment.  The  wish  is  often  father 
to  the  thought — physical  discomfort  or  other  causes  may  instil 


12  MINE    SAMPLING   AND   VALUING 

a  restlessness  or  perhaps  a  hope  that  a  preliminary  examination 
may  give  sufficient  information  for  an  unfavorable  report.  We 
can  reason  out  any  desired  conclusion,  if  we  but  assume  a  suitable 
hypothesis.  An  examining  engineer  must  keep  an  open  mind 
and  must  not  form  a  premature  judgment  based  on  insufficient 
data.  He  must  be  a  mere  collector  of  facts  until  it  is  time  to  form 
a  decision.  An  assay  plan  has  changed  many  preconceived 
notions.  The  facts  must  be  collected  and  put  away  in  the  proper 
pigeon-hole  of  one's  mind  until  required,  for  if  the  engineer  is 
biased,  either  consciously  or  unconsciously,  such  bias  will  be  a 
factor  in  forming  the  ultimate  opinion. 

One  often  hears  the  question  asked  whether  a  report  has  been 
made  for  a  seller  or  a  buyer,  indicating  a  possibility  that  the  en- 
gineer's judgment  may  be  affected  by  the  character  of  his  em- 
ployment. I  wish  tojnake  a  strong  plea  for  a  standard  of  action 
among  engineers,  for 'a  mode  of  procedure  such  as  is  customary, 
for  instance,  in  the  medical  profession.  An  engineer  making  a 
mine  examination  should  view  his  work  in  the  same  light  that  a 
doctor  does  a  patient  and  should  make  a  diagnosis,  regardless  of 
any  outside  consideration.  As  engineers  we  cannot  provide 
against  the  moral  turpitude  of  individuals,  but  at  least  we  can 
adopt  a  standard  of  action  that  conscientious  men  may  follow,  so 
as  to  destroy  the  evil  effects  of  mining  charlatans  and  adventurers, 
who  happily  do  not  flourish  so  profusely  now  as  formerly,  and 
thus  raise  the  profession  in  the  eyes  of  the  general  public  to  the 
position  it  deserves. 

Before  proceeding  further,  it  is  advisable  to  define  the  word 
'ore'  as  it  will  be  used  in  this  discussion  and  as  the  author  believes 
it  is  used  by  most  mining  engineers.  J.  F.  Kemp,  professor  of 
geology  at  Columbia  University,  in  a  paper  presented  to  the 
Canadian  Mining  Institute,1  after  quoting  most  of  the  definitions 
of  ore  then  extant  and  discussing  them,  defined  ore  as  follows : 

"In  its  technical  sense  an  ore  is  a  metalliferous  mineral  or  an 
aggregation  of  metalliferous  minerals,  more  or  less  mixed  with 
gangue  and  capable  of  being,  from  the  standpoint  of  the  miner, 
won  at  a  profit,  or  from  the  standpoint  of  the  metallurgist,  treated 
at  a  profit." 

This  definition  seems  to  be  too  restrictive  when  judged  from 
the  .ordinary  mining  standpoint.  Personally,  I  prefer  either  of 
the;,' following  conceptions,  the  first  one  being  a  modification  of 
that  appearing  in  Murray's  'New  English  Dictionary'  (1908),  and 
the  second  by  R.  H.  Stretch. 

(1).  "A  native  mineral  or  aggregation  of  minerals  containing 
one  or  more  precious  or  useful  metals  in  such  quantity  and  in  such 
chemical  combination  as  to  make  its  extraction  profitable."  (The 
italics  show  the  changes  from  Murray's  definition.) 

(2).  "An  'ore,'  strictly  speaking,  is  a.  single  mineral  which  is 
a  chemical  compound  of  a  useful  metal  and  some  other  element 

1  Mining   and   Scientific  Press,   vol.    98;    p.   419. 


GENERAL    PRINCIPLES  13 

or  acid.  In  common  usage,  however,  complex  mixtures  of  pure 
minerals  are  considered  as  single  ores;  while  free  gold,  native 
silver,  and  native  copper,  together  with  their  accompanying 
gangue  minerals,  are  also  classed  as  ore.  Among1  miners  whatever 
will  pay  to  treat  or  ship  and  sell,  is  considered  ore,  as  also  low-grade 
mineral,  which  might  be  utilised  by  concentration  or  improved  facil- 
ities; but  there  is  an  indefinite  shading  off  into  material  containing 
traces  of  ore-minerals  but  hopelessly  unavailable,  and  this  is  not  con- 
sidered ore;  neither  are  gold  gravel  or  platinum  sand  called  ore." 

Stretch  emphasizes  the  shading  off  of  value  indicating  the 
general  usage  of  mining1  men,  that  not  until  the  percentage  of 
valuable  mineral  is  practically  negligible  is  the  term  dropped. 

The  generally  accepted  definition  among  both  technically 
trained  and  non-technical  mining  men  is  that  it  is  used  to  desig- 
nate a  metallic  mineral  or  minerals  occurring  in  such  quantity 
which  we  know  from  experience  to  be  of  commercial  value.  In 
other  words,  the  miner  (i.  e.  one  who  mines)  is  continually  dis- 
counting the  improvements  in  science  and  by  common  accord 
designates  as  ore,  not  only  material  which  can  be  made  to  yield 
a  profit  at  the  moment,  but  also  such  as  contains  appreciable 
quantities  of  the  valuable  mineral  being  exploited,  as  distinguished 
from  the  purely  barren  gangue  and  the  country  rock. 

In  discussing  Kemp's  paper,  I  suggested  the  following  defini- 
tion in  place  of  the  one  offered  by  him : 

"An  ore  is  a  metalliferous  mineral  or  an  aggregation  of 
metalliferous  minerals,  more  or  less  mixed  with  gangue,  that  from 
the  standpoint  of  the  metallurgist  can  be  treated  at  a  profit,  or 
that  from  the  standpoint  of  the  miner  occurs  in  a  vein,  lode  or 
other  geological  deposit  and  concentrated  by  nature  in  such  a 
manner  as  to  attract  the  attention  of  the  miner  on  account  of  his 
belief  that  he  can  work  at  least  a  portion  of  such  deposit  at  a 
profit."2 

I  am  in  agreement  with  Kemp  so  far  as  the  metallurgist's 
viewpoint  is  concerned,  but  differ  in  other  respects.  I  believe 
however  that  my  definition  conforms  to  the  view  most  usually 
held  by  mining  men. 

Many  engineers'  reports  nowadays  state  not  only  the  tonnage 
of  ore  amenable  to  profitable  treatment  under  the  conditions  exist- 
ing at  the  time  of  their  examination,  but  likewise  give  the  tonnage 
and  value  of  'low-grade  ore'  that  experience  teaches  may  reasonably 
be  expected  to  lend  itself  to  profitable  handling  at  a  later  date; 

All  mining  operations  are  conducted  with  a  view  to  earning  a 
profit,  hence  that  question  cannot  be  eliminated,  neither  does  it 
limit  the  use  of  the  word  among  the  men  whose  use  makes  or  un- 
makes its  meaning.  No  man  starts  a  business  of  any  kind  unless 
he  expects  to  make  a  profit,  but  not  all  business  is  done  at  a  profit 
In  attempting  to  apply  the  standard  of  profit  to  the  use  of  the 
word  'ore,'  its  advocates  are  placing  a  restrictive  meaning  on  it 

"Mining   and   Scientific  Press,   vol.    99;    p.    117. 


14  MINE    SAMPLING   AND   VALUING 

that  is  neither  understood  nor  applied  by  the  professional  man  nor 
yet  the  practical  miner.  For  instance,  Rickard  defines  ore  as 
metal-bearing  rock,  which  at  a  given  time  and  place  can  be  ex- 
ploited at  a  profit. 

Mine  No.  1  may  be  making  a  profit,  while  its  neighbor  work- 
ing the  same  ore  makes  a  loss,  due  to  bad  management  or  inade- 
quate plant.  It  is  absurd  to  designate  one  as  ore  and  the  other 
as  waste.  Because  the  business  is  run  at  a  loss  does  not  deprive 
it  of  its  commercial  aspect.  Just  as  in  any  other  branch  of  com- 
merce, the  idea  of  profit  is  present  in  all  mining  operations  and  is 
the  chief  incentive,  but  failure  to  attain  the  sought-for  end  does 
not  change  the  character  or  name  of  the  commodity.  So  with  the 
word  'ore,'  if  by  the  time  the  metal  produced  is  sold  in  the  market 
and  a  loss  is  shown  in  the  operations,  the  metal-bearing  rock  from 
which  it  was  derived  is  none  the  less  'ore.' 

Let  us  consider  another  aspect  of  the  case.  No  doubt  "use  is 
the  law  of  language"  and  except  for  the  small  circle  who  are  at- 
tempting to  restrict  the  meaning  to  the  standard  of  profit,  the 
meaning  of  the  word  'ore'  is  pretty  generally  understood  through- 
out the  mining  world.  If  one  speaks  of  ore  and  knows  the  mine, 
there  is  no  possibility  of  misunderstanding  what  is  meant.  If  the 
word  'ore'  is  restricted  to  metal-bearing  rock  that  can  be  exploited 
at  a  profit  and  everything  else  is  waste,  then  we  are  confronted 
with  the .  necessity  of  coining  another  expression  for  the  inter- 
mediate product  between  ore  and  the  absolutely  barren  country 
rock  or  barren  gangue  material. 

To  give  a  concrete  case,  assume  an  orebody,  consisting  of  three 
distinct  kinds  of  material,  namely : 

1.  Ore  that  will  yield  a  profit. 

2.  Ore  which  is  too  low  grade  to  yield  a  profit. 

3.  Absolutely  barren  quartz,  or  other  similar  worthless  gangue. 

Where  these  materials  occur  in  distinct  bunches,  shoots,  zones, 
bands  or  other  manner,  mining  operations  will  be  carried  on  with 
a  view  to  breaking  only  class  No.  1,  namely,  the  ore  that  will  yield 
a  profit.  Class  No.  2  may  with  better  economic  conditions,  such 
as  improved  metal  prices,  cheaper  supplies,  etc.,  become  profitable 
at  any  time. 

No  mining  man  would  use  the  same  expression  to  denote  Class 
2  and  Class  3  and  would  have  no  hesitancy  in  speaking  of  barren 
gangue  and  country  rock  as  waste  at  any  time,  regardless  of  the 
economic  conditions,  or  metal  prices.  No  one  would  hesitate  to 
use  rock  of  this  sort  as  filling  or  for  road  metal,  or  for  any  other 
similar  purpose,  where  it  is  beyond  recovery.  On  the  other  hand, 
the  materials  comprising  Class  2  would  be  carefully  guarded  and 
certainly  would  not  be  called  waste.  If  it  is  not  ore,  then  another 
name  must  be  given  to  it,  which  we  would  have  to  learn.  With 
the  present  use  of  the  expressions  'profitable  ore'  and  'unprofitable 
ore'  there  can  be  no  misinterpretation  and  certainly  these  expres- 
sions are  as  suitable  as  any  new  ones  that  may  be  coined. 


GENERAL    PRINCIPLES  15 

Therefore,  the  sampling  of  a  mineral  deposit  must  be  con- 
ducted with  a  view  not  only  of  determining  the  tonnage  and  value 
of  the  profitable  ore,  but  the  tonnage  and  value  of  low-grade  or 
unprofitable  ore  as  well,  so  far  as  the  conditions  justify  the  necessary 
expenditure.  The  miner  is  first  of  all  concerned  with  the  gross 
value  of  the  metallic  content.  Fluctuating  prices  of  metals,  or 
efficiency  of  metallurgical  treatment,  which  may  govern  the  ques- 
tion of  profit,  do  not  restrict  his  use  of  the  word  ore. 

Mining  engineers  are  most  particularly  occupied  with  the  ex- 
ploitation of  rocky  masses  occurring  as  veins  or  orebodies  sur- 
rounded by  rock  usually  of  a  different  character,  in  any  case  so 
far  different  as  to  be  differentiated  as  waste  rock.  From  the  very 
nature  of  their  occurrence  these  orebodies  usually  extend  to  a 
relatively  considerable  depth  below  the  surface  of  the  ground. 
Therein  are  presented  some  of  the  inherent  difficulties  attendant  on 
the  work.  Underground  workings  are  required  for  their  exploita- 
tion which  are  usually  dimly  illuminated,  wet,  cramped,  and 
obstructed  by  timber.  The  following  pages,  therefore,  deal  with 
the  sampling  and  estimation  of  mineral  deposits  in  situ  opened 
by  underground  methods;  not  excluding,  however,  low-grade 
copper,  iron,  or  similar  deposits  that  for  economic  reasons  are  being 
mined  by  steam-shovels  or  other  mechanical  appliances  and  open 
to  daylight. 

In  a  consideration  of  the  subject  of  mine  sampling,  it  must  be 
borne  in  mind  that  the  sampling  of  a  mine  by  an  independent 
engineer  for  valuation  purposes  is  a  different  problem  than  the 
periodical  estimation  of  ore  by  the  mine  management  for  reports 
to  directors  and  shareholders.  The  latter  involves  no  money 
transaction  and  the  same  care  is  not  required  nor  is  the  responsi- 
bility so  great.  The  independent  engineer  uses  the  utmost  care 
and  sacrifices  all  other  things  to  accuracy.  In  the  daily  sampling 
of  a  mine  the  first  concern  of  the  management  must  necessarily  be 
utility  and  expediency;  as  a  consequence,  the  daily  mine  samples 
are  individually  apt  to  vary  considerably  from  the  true  value.  In 
practice  it  is  usual  to  apply  an  empirical  factor  of  correction  as 
giving  sufficiently  accurate  results.  The  present  discussion  is  more 
particularly  concerned  with  the  aspects  of  mine  sampling  for  valu- 
ation purposes,  because  on  operating  mines  the  management  has 
time  to  work  out  the  proper  factor  to  meet  the  existing  conditions. 
On  the  other  hand,  the  visiting  engineer  must,  in  the  short  space 
of  time  occupied  by  his  examination,  use  methods  that  will  ensure 
the  desired  accuracy. 


CHAPTER  II. 

PICK  ANALYSIS. 

It  is  bad  practice  to  commence  sampling  operations  before  a  knowl- 
edge of  the  deposit  has  been  obtained  by  means  of  a  study  of  its 
mineralogical  and  geological  character,  both  on  surface  and  under- 
ground, and  this  work  is  what  I  have  termed  'pick  analysis.' 

It  is  a  curious  fact  that  in  the  entire  literature  on  the  subject  of 
mine  sampling  and  valuation,  no  attention  has  ever  been  called  to  this 
work,  nor  does  it  seem  to  be  the  usual  custom  of  most  examining  engi- 
neers. The  general  method  of  procedure  with  many  engineers  is  to 
take  a  few  scattered  samples  as  a  preliminary  step  in  order  to  locate 
the  occurrence  of  values.  This  method  can  only  be  described  as  hap- 
hazard, and  is  not  in  line  with  the  systematic  methods  desirable  in  such 
important  work  as  mine  examination.  Even  with  the  simplest  kind 
of  deposit,  a  systematic  investigation  should  be  made  to  determine  its 
mineralogical  and  geological  characteristics.  In  other  words,  we  bring 
into  the  realm  of  mining  engineering  some  of  the  methods  employed 
by  the  geologist,  but  with  a  different  object.  Taking  a  few  prelimi- 
nary samples  is  not  a  sufficient  guide  for  conducting  comprehensive 
sampling  operations  intelligently,  nor  is  the  more  customary  practice  of 
merely  walking  through  the  mine  enough. 

Very  often  the  sampling  of  a  mine  is  started  immediately,  before  any 
detailed  knowledge  of  the  deposit  has  been  acquired,  and  this  is  akin 
to  a  man  walking  blindfolded.  The  ore  exposures  underground  are 
usually  besmudged  by  dirt,  water,  and  powder  smoke,  which  makes 
them  difficult  to  examine,  and  thereore  it  is  necessary  to  obtain  a  clean 
surface,  or  by  chipping  off  a  piece  of  the  face  get  a  fresh  fracture,  free 
from  surface  oxidation.  Usually  the  mine  foreman  has  a  fairly  accu- 
rate idea  of  the  mode  of  ore  occurrence  and  distribution  of  values, 
for  the  simple  reason  that  he  watches  the  development  work  from  day 
to  day,  sees  the  faces  after  each  round  of  shots,  and  thereby  is 
enabled  not  only  to  study  the  geological  structure  but  the  mineralogical 
character  of  the  orebody  as  well.  Mine  foremen  are  scarcely  expert 
geologists,  but  observation  teaches  them  that  the  occurrence  of  valuable 
minerals  in  any  particular  deposit  is  usually  accompanied  by  certain 
specific  conditions  or  indications. 

The  examining  engineer,  in  order  to  properly  sample  the  mine  and 
afterwards  to  interpret  intelligently  the  results,  must  familiarize  him- 
self with  these  facts.  He  must  rapidly  acquire  the  knowledge  that  it 
has  taken  the  mine  foreman  months  or  years  to  learn,  and  it  must  be 
done  by  optical  inspection. 

As  a  preliminary,  the  available  mine  maps  should  be  studied  and 
the  mine  officials  thoroughly  interrogated  regarding  all  questions  of 
interest.  In  this  way  a  fairly  exact  knowledge  of  the  conditions  may 


PICK    ANALYSIS  17 

often  be  acquired  before  going  underground.  The  ore  and  waste 
dumps  should  be  examined  to  familiarize  oneself  in  daylight  with  the 
character  of  the  ore  and  rock  found  in  the  workings.  This  done,  the 
engineer  should,  in  company  with  one  of  the  mine  officials,  make  a 
cursory  inspection  underground,  to  enable  him  to  get  his  bearings. 
Henceforward  he  should  continue  the  investigation  without  the  pres- 
ence of  any  person  other  than  those  in  his  employ. 

Before  sampling  is  attempted,  the  detailed  optical  inspection,  or 
pick  analysis,  of  the  exposed  faces  in  the  workings  should  be  made  by 
chipping  the  ore  and  rock  exposures  at  short  intervals  in  the  drifts, 
cross-cuts,  and  other  workings,  by  means  of  a  prospecting  pick  or  a 
miner's  pick,  if  the  ground  be  very  hard,  until  a  thorough  familiarity 
with  the  characteristics  of  the  orebody  is  acquired.  The  country  rock 
and  dikes  should  receive  attention,  but  the  same  thoroughness  is  not 
required  with  them  except  where  necessary  to  solve  some  geological 
question.  By  this  means  there  will  be  determined  the  general  conditions 
as  to  character  of  mineralization  and  the  amount  of  sulphides,  base 
metals  or  associated  minerals  present,  the  width  of  the  orebody,  whether 
regular  or  irregular,  the  existence  of  lenses  or  shoots,  the  presence  of 
faults  and  dikes  and  their  effect  on  the  position  of  the  orebody  and  in 
some  cases  their  influence  on  the  grade  of  the  ore.  This  inspection 
should  determine  the  portions  of  the  orebody  to  be  sampled  and  those 
that  can  be  regarded  as  worthless ;  in  other  words,  determine  the  limits 
of  the  mineralized  zone.  With  this  knowledge  in  hand  intelligent 
sampling  operations  can  be  undertaken. 

The  usefulness  of  pick  analysis  can  be  better  appreciated  by  citing 
a  concrete  case.  A  few  years  ago  a  copper  mine,  held  under  option 
at  a  high  price,  was  examined.  The  property  was  at  a  considerable 
distance  from  a  railroad,  although  a  connecting  line  was  contemplated, 
and  the  district  was  one  where  labor  was  scarce  and  costly.  The 
previous  history  of  the  mine  was  that  a  considerable  quantity  of  ore 
had  been  mined  and  smelted  on  the  spot.  Two  assistants  chipped  the 
faces  of  the  hard  rock  for  my  inspection,  so  that  I  was  enabled  by  this 
means  to  gather  sufficient  evidence  in  two  days  to  warrant  an  adverse 
report. 

By  optical  inspection  a  close  approximation  of  the  average  cop- 
per content  was  made,  to  check  the  figures  given  in  the  reports,  and 
a  simple  calculation  demonstrated  that  even  allowing  all  the  content 
in  the  ore  claimed  for  it  by  the  vendors,  there  was  not  enough  tonnage 
available  to  warrant  the  price.  The  mine  was  at  a  different  time  ex- 
amined by  another  engineer  who  spent  three  or  four  weeks  sampling 
it,  to  finally  show  something  like  15,000  tons  of  3  to  4%  ore  in  sight. 
So  he  turned  it  down.  He  might  have  been  saved  considerable  time 
and  expense  by  having  followed  the  other  method.  Pick  analysis  will 
often  determine  the  fact  that  in  the  bottom  level  of  the  mine  the  valu- 
able minerals  are  'petering  out.'  By  first  sampling  the  bottom  level, 
sufficient  evidence  may  be  gained  to  obviate  the  necessity  of  any 
other  work.  No  one  wants  to  buy  a  mine  with  a  bad  bottom,  as  a 
diminution  of  value  generally  indicates  that  the  limits  of  the  orebody 


18  MINE    SAMPLING    AND   VALUING 

are  being  approached.  Pick  analysis  will  almost  always  indicate  the 
existence  of  high-grade  streaks  and  their  mode  of  occurrence,  but 
above  all  else,  it  gives  the  engineer  a  thorough  knowledge  of  the  mode 
of  ore  occurrence,  character  of  mineralization,  etc.,  so  essential  in 
conducting  intelligent  sampling  operations.  This  work  should  never 
be  delegated  to  subordinates,  but  must  be  done  by  the  responsible 
engineer  himself. 

In  the  case  of  a  gold-bearing  quartz,  with  little  or  ho  associated 
sulphide  minerals,  the  information  obtained  is  not  so  comprehensive, 
as  it  is  in  the  case  where  sulphides  occur  in  appreciable  quantity,  or 
in  the  case  of  base'  metal  mines,  especially  those  of  copper,  lead  and 
zinc,  where  often  with  a  little  experience  the  assay  value  may  be 
closely  approximated  by  mere  optical  inspection. 

Be  the  ore  what  it  may  and  whatever  its  genesis  or  type,  a  thorough 
investigation  by  pick  analysis  will  greatly  facilitate  not  only  the  subse- 
quent sampling  operations  but  will  be  of  value  in  the  ultimate  work 
of  estimating  the  ore  reserves.  It  is  usually  a  tedious  job,  but  one 
that  will  well  repay  the  effort. 


CHAPTER  III. 

IMPLEMENTS  EMPLOYED  IN  SAMPLING. 

A  sampling  outfit  must  consist  not  only  of  tools  for  breaking  the 
rock,  but  also  means  to  safeguard  samples  from  interference  after 
they  have  been  taken.  Certain  additional  paraphernalia  are  required, 
as  set  forth  in  the  accompanying  list,  which  is  recommended  as  a  re- 
sult of  my  own  experience. 

List  of  Implements. 

Hammers. 

Moils  and  (or)  gads,  and  (or)  chisels. 

Miner's  pick. 

Prospecting  pick. 

Shovel. 

Whisk-broom,  scrub-brush  or  similar  article. 

Steel  tape  50  or  100  ft.  « 

Steel  tape  10  ft. 

Canvas  sheets. 

Duck  sampling  buckets. 

Leather  sack  with  Yale  lock. 

Small  sample  sacks. 

Twine. 

Seal  and  sealing  wax. 

Linen  baggage  labels. 

Pencils. 

Plumb  bobs  and  lines. 

Pocket  compass  (preferably  Brunton). 

Whitewash. 

Jones  sampler  or  other  mechanical  divider. 

Paper  sample  envelopes. 

HAMMERS. — Cutting  a  sample  is  different  from  cutting  a  hitch  for 
timber  and  therefore  the  hammer  should  not  be  of  excessive  weight. 
As  the  force  of  the  blow  is  regulated  by  the  work  to  be  done,  the 
hammer  should  be  thoroughly  under  the  control  of  the  user.  In  the 
machine  shop  there  is  a  parallel  case ;  the  machinist's  hammer  for  cut- 
ting steel  and  iron  rarely  weighing  more  than  two  pounds.  Using  a 
four-pound  hammer  underground  after  a  period  of  inactivity  soon 
wearies  the  muscles  and  lessens  control.  This  causes  the  operator  to 
miss  the  head  of  the  tool  and  means  bruised  and  skinned  hands.  My 
own  practice  is  to  use  an  ordinary  double-head,  single-hand  hammer 
with  a  broad  face  and  weighing  3  to  3^  Ib.  for  cutting  the  sample, 
except  where  the  ground  is  extremely  hard  and  a  double-hand  ham- 
mer becomes  necessary.  The  larger  hammers  should  weigh  about 
7  Ib.  and  are  otherwise  used  for  breaking  rock. 


20  MINE    SAMPLING   AND   VALUING 

MOILS,  GADS  and  CHISELS— Should  be  of  convenient  size 
and  weight.  This  is  regulated  by  the  diameter  of  the  steel  used  and 
the  length  of  the  tool.  Inexperienced  people  often  make  the  mistake 
of  employing  heavy  steel.  It  must  be  sufficiently  stout  to  withstand 
the  hardness  of  the  ground,  but  as  light  as  possible  commensurate 
with  the  work  to  be  done.  Heavy  steel  has  the  double  disadvantage 
of  excessive  weight  and  so  large  cross-section  as  to  absorb  too  much 
of  the  force  of  the  blow.  The  most  useful  size  is  ^-in.  octagonal 
drill  steel,  although  5^-in.  steel  may  be  used  occasionally,  where  cir- 
cumstances permit. 

Weight  is  an  important  factor,  as  often  the  sampler  is  in  a  cramped 
position,  standing  on  a  scaffold,  box,  saw-horse,  or  other  unstable 
object  with  muscles  tensed,  having  to  support  the  entire  weight  of  the 
steel  in  his  hand,  watch  the  sample,  and  leave  room  for  his  assistant 
holding  the  bucket.  Under  such  circumstances  a  heavy  tool  may 
cause  so  much  discomfort  as  to  seriously  retard  the  sampling  opera- 
tions. Moiling  is  tedious  work  and  exactitude  is  required,  therefore 
the  steel  should  weigh  as  little  as  consistent  with  the  duty  expected 
of  it,  thus  leaving  the  sampler  free  to  put  all  his  force  into  the  ham- 
mer blow. 


I 


IMPROPER  WAY  OF  SHARPENING. 


o 


Fig.   1.     PROPER  WAY  OF  SHARPENING. 

The  moil  is  more  generally  employed  than  the  gad  or  chisel,  as 
the  sharp  point  will  usually  get  a  bite  in  the  rock  quicker  than  the 
straight  edge  of  the  gad.  In  sharpening  a  moil,  instead  of  drawing 
the  point  down  from  the  shank  of  the  steel  in  a  simple  pyramid,  the 
so-called  diamond  point  is  preferable,  that  is,  the  extreme  point  should 
be  a  small  flat  pyramid  terminating  the  larger  one,  thereby  strength- 
ening the  tool  where  it  is  most  needed  by  presenting  a  greater  cross 
section  of  metal.  One  of  the  greatest  difficulties  in  moiling  rock  is 
the  frequency  with  which  the  point  breaks  off.  By  this  method  of 
sharpening,  the  moil  is  strengthened  and  less  breakage  occurs.  See 
Fig.  1. 

The  gad  or  wedge  is  an  ancient  tool.  It  is  fitted  with  a  handle 
and  is  one  of  the  two  tools  comprising  the  emblem  known  as  the 
crossed  hammers.  It  is  primarily  intended  for  breaking  out  rock  by 
the  force  of  the  wedge. 

The  chisel  may  often  be  usefully  employed,  especially  in  sampling 
slate  or  schist.  The  tool  for  sampling  is  sharpened  just  like  the  ordi- 
nary machinist's  cold  chisel,  and  in  slate  or  schist  has  the  advantage 
of  cutting  a  more  regular  channel  with  the  same  effort. 

1 


IMPLEMENTS    EMPLOYED    IN    SAMPLING  21 

Care  should  be  exercised  in  tempering  these  tools,  the  best  color 
being  a  light  pigeon  blue  or  slightly  darker.  This  affords  the  requisite 
toughness  for  hard  rock,  and  at  the  same  time  avoids  the  brittleness 
of  a  harder  temper.  If  the  ground  be  extremely  hard,  better  results 
can  be  obtained  by  tempering  in  oil. 

A  variety  of  lengths  should  be  supplied  and  in  hard  ground  twenty 
to  thirty  moils  per  day  may  be  needed.  The  most  useful  length,  how- 
ever, is  10  to  12  inches.  Where  the  roof  or  face  is  somewhat  out 
of  reach  of  the  sampler,  longer  steel  can  be  usefully  employed,  but 
experience  teaches  that  the  shorter  moil  tends  to  greater  accuracy. 
It  permits  close  inspection  of  the  cut  while  the  sample  is  being  taken, 
gives  a  more  direct  application  of  the  hammer  blow,  and,  as  previously 
mentioned,  the  light  weight  is  an  important  factor  to  the  ordinary 
engineer  not  accustomed  to  manual  labor. 

PICK  and  SHOVEL. — These  may  be  of  the  ordinary  type  used 
about  the  mine,  although  the  poll  pick  is  preferable.  The  prospector's 
pick  may  weigh  \y2  to  2  pounds. 

STEEL  TAPES. — The  long  tape  is  used  for  marking  out  the 
sampling  intervals  and  such  other  measurements  as  are  necessary  in 
the  work,  apart  from  surveying.  A  Chesterman  or  other  first-class 
50  or  100- ft.  steel  tape,  divided  into  feet  and  tenths,  should  be  used, 
and  in  no  instance  should  a  linen  tape  be  employed  in  sampling  opera- 
tions. The  short  tape  should  be  of  the  spring  type,  so  as  to  permit 
the  sampler  to  extend  or  shorten  it  with  one  hand.  This  is  a  small 
point  but  may  be  the  means  of  saving  considerable  time  and  annoy- 
ance, as  a  reel  tape  requires  the  use  of  both  hands  and  occasionally 
when  standing  on  a  scaffold  or  shaky  box,  holding  a  candlestick  at 
the  same  time,  even  this  small  operation  may  become  extremely  irk- 
some. 

CANVAS  CLOTH. — A  canvas  sheet  5  or  6  feet  square  is  re- 
quired as  a  mixing  cloth,  as  well  as  several  pieces  3  ft.  square.  En- 
quiries made  in  England  a  few  years  ago  show  that  canvas  6  ft.  in 
width  is  not  manufactured  in  England,  although  it  is  easily  obtain- 
able in  the  United  States.  Such  being  the  case,  engineers  in  the 
former  country  will  of  necessity  have  to  have  two  widths  sewn  to- 
gether. 

DUCK  SAMPLING  BUCKET.— For  fifteen  years  I  have  used  a 
duck  or  canvas  bucket  in  preference  to  a  box  or  other  receptacle  or  to 
the  large  canvas  sheet  so  often  employed  for  collecting  the  chips  of 
ore  broken  from  the  face  to  form  the  sample.  In  my  opinion  these 
buckets  have  many  advantages  over  any  other  thing  that  is  ordinarily 
used  for  this  purpose.  The  unconscious  salting  of  a  sample  where 
brittle  sulphides  occur  is  minimized,  and  the  spoiling  of  a  sample  by 
a  fall  of  ground,  so  common  in  sampling  work,  is  practically  elimi- 
nated. 

One  of  these  buckets  weighs  but  a  few  ounces  as  compared  with 
the  considerable  weight  of  a  wooden  box.  The  strain  on  one's  mus- 
cles in  holding  a  candle  box  in  the  position  shown  in  the  frontispiece 
of  T.  A.  Rickard's  book  on  the  'Sampling  and  Estimation  of  Ore  in  a 


22 


MINE    SAMPLING   AND   VALUING 


Mine'  can  easily  be  recalled  by  anyone  who  has  tried  it.  There  are 
many  places  in  a  mine  where  the  receptacle  must  be  held  at  arm's 
length  overhead,  and  the  mere  position  is  sufficiently  tiring  without 
sustaining  anything  heavy.  When  the  weight  of  the  sample  is  added 
to  the  already  heavy  box,  the  work  becomes  irksome  indeed,  and  a 
slight  shifting  of  the  box,  as  the  assistant's  muscles  get  tired,  may 
cause  the  sampler  to  bash  his  knuckles  and  visit  his  wrath  upon  the 
head  of  the  poor  fellow  holding  the  box.  As  compared  with  this,  the 
canvas  bucket  not  only  has  the  advantage  of  greater  lightness,  but  in 
view  of  its  flexibility,  permits  the  assistant  to  more  intelligently  follow 
the  operations  of  the  sampler  and  thereby  facilitate  the  work.  Should 
the  sampler  miss  hitting  the  moil,  it  is  infinitely  less  aggravating  to 
strike  a  duck  bucket  with  one's  knuckles  than  the  sharp  edge  of  a 
candle  box. 


Fig.  2.  CANVAS  SAMPLING  BUCKET. 


Fig.  3.  A  BUCKET  THROTTLED  FOR 
SOFT  GROUND. 


In  soft  ground  where  the  ore  is  apt  to  come  away  in  chunks,  the 
bucket  can  be  throttled  in  the  middle  by  the  hand,  so  as  to  form  two 
compartments  and  if  an  excessive  amount  does  come  away  the  excess 
can  be  rejected  before  the  remainder  is  added  to  the  sample.  With 
a  candle  box  under  the  same  circumstances,  either  the  sample  would 
have  to  be  discarded  entirely,  or  an  inaccurate  sample  taken.  See 
Fig.  2. 

The  bucket  is  about  9  in.  diameter  and  14  in.  deep.  Suitable 
buckets  can  often  be  purchased  from  camp  outfitters.  If  necessary 
they  can  be  made  in  any  small  town  where  duck  or  canvas  is  obtain- 
able. Any  blacksmith  can  make  the  ring,  which  should  be  of  3/16- 
in.  iron,  over  which  the  duck  is  sewed  in  such  a  manner  that  all  the 
seams  are  outside.  The  long  seam  down  the  side  should  be  a  lap 
seam  similar  to  the  seam  in  a  sail,  without  any  rough  edge,  the 
selvedge  if  possible  being  on  the  inside  of  the  bucket.  The  bottom 
should  be  put  in  with  the  seam  outside,  the  rough  edges  bound  with 


IMPLEMENTS    EMPLOYED    IN    SAMPLING 


23 


tape  or  thin,  pliable  leather  to  give  a  neater  appearance  and  to  pre- 
vent the  unraveling  of  the  fabric.  A  handle  of  Y^-'m.  rope  will  be 
found  a  convenience.  See  Fig.  3. 

The  ring  forming  the  mouth  of  the  bucket  is  sometimes  made  of 
copper  wire.     The  ends  can  be  joined  by  brazing  a  sleeve  over  them 


Fig.    4.     LEATHER    SACK    FOR    CARRYING    SAMPLES. 

or  they  can  be  simply  doubled  over  themselves.  The  use  of  the 
wire  permits  shaping  the  ring  by  hand  in  such  a  manner  as  to  allow 
the  bucket  to  be  held  closer  to  the  face.  In  my  opinion  this  ar- 
rangement is  of  doubtful  advantage.  This  wire  need  not  be  of 
such  large  diameter  as  the  iron. 

The  LEATHER  SACK.— A  leather  sack   (see  Fig.  4),  about  30 
by  18  in.  or  26  by  15  in.,  in  the  form  of  a  United  States  mail  sack 


24  MINE    SAMPLING    AND   VALUING 

and  fitted  with  a  Yale  lock,  should  be  carried  for  safeguarding  sam- 
ples both  underground  and  on  the  way  from  the  workings  until  they 
can  be  placed  securely  under  lock  and  key  on  the  surface.  The  use 
of  such  a  leather  sack  is  almost  a  sure  preventive  against  salting  at 
this  period  of  the  operations.  Salting  to  be  well  done  necessitates 
that  the  salter  sees  what  he  is  doing,  so -that  he  can  increase  the  gold 
contents  of  each  individual  sample  by  a  desired  amount. 

Once  locked  up  in  the  mail  sack,  the  only  way  of  getting  at  the 
samples  to  salt  them  is  by  making  an  incision  in  the  leather  and  in- 
jecting a  solution  into  the  sample.  As  only  a  portion  of  the  total, 
number  of  samples  can  be  reached  in  this  way,  proper  salting  is  inter- 
fered with,  and  as  the  salter  is  working  in  the  dark  such  erratic 
assay  results  are  likely  to  be  obtained  as  to  lead  to  immediate  investi- 
gation and  detection.  An  incision  made  in  the  leather  may  escape 
notice,  but  the  imposture  would  be  discovered  by  the  engineer  as 
soon  as  he  arrived  on  the  surface,  where  it  should  be  his  immediate 
duty  to  take  the  samples  from  the  mail  sack  and  place  them  in  safety 
in  the  assay  office,  box,  or  whatever  may  be  his  method  of  safeguard- 
ing them  until  ready  for  assaying.  While  making  this  transfer  of 
samples,  the  excessive  wetness  of  the  sacks,  where  the  solution  has 
been  injected,  will  generally  be  readily  apparent,  even  if  there  were 
no  means  of  discovering  the  salting  during  the  subsequent  operations. 

In  the  United  States  these  sacks  can  be  purchased  ready-made 
from  leather  goods  manufacturers  in  various  parts  of  the  country ;  in 
England  they  can  be  made  by  any  good  saddler.  The  sack  should  be 
made  of  stout  leather,  the  side  seam  secured  by  copper  rivets  spaced 
close  together  so  as  to  make  the  joint  secure.  The  bottom  should 
be  riveted  to  the  sides  with  a  lap  joint,  the  overlapping  ends  being 
bound  with  leather. 

There  should  be  a  handle  both  top  and  bottom  and  two  loops  on 
either  side  big  enough  to  allow  a  rope  to  be  passed  through  them, 
so  that  the  sack  may  be  securely  fastened  to  a  pack  saddle.  Very 
often  these  sacks  are  the  only  means  the  engineer  has  for  carrying 
his  mine  samples  in  mountainous  districts  and  the  loops  are  of  great 
assistance  when  pack  animals  are  the  only  means  of  transport. 

SAMPLE  SACKS. — Two  sizes  of  sample  sacks  are  ordinarily 
used,  the  smaller  6  by  12  in.,  and  the  larger  9  by  14  in.  They 
should  be  made  of  duck,  or  light-weight  canvas,  double  sewn  with 
strong  linen  thread,  and  should  always  be  kept  under  lock  and  key 
to  prevent  tampering.  When  this  formality  has  been  neglected,  the 
sacks  are  turned  inside  out  and  thoroughly  beaten  before  use.  When 
in  use  the  seams  should  be  inside. 

SUNDRY  ARTICLES.— Abundant  sealing  wax  of  first-class 
quality  should  be  provided  and  a  well  cut  and  distinctive  seal  about  y<\ 
or  1  in.  diameter  used  to  seal  the  sacks.  Lead  seals  are  sometimes 
used  instead  of  wax,  but  they  are  cumbersome  and  give  no  better  re- 
sult. Besides,  sealing  wax  can  be  purchased  almost  anywhere. 

Linen  baggage  labels  of  a  small  size  are  the  best  for  marking 
samples,  as  rough  handling  does  not  destroy  them.  Slips  of  paper 


IMPLEMENTS    EMPLOYED    IN    SAMPLING 


25 


put  in  wet  ore  or  coarse,  sharp,  hard  ore  are  apt  to  become  mutilated 
and  illegible.  • 

An  indelible  pencil  should  be  used  for  marking  the  sample  num- 
bers. A  hand  compass  is  a  necessity  underground  for  taking  bear- 
ings. Plumb-lines  form  a  useful  adjunct.  Whitewash  can 
usually  be  obtained  at  the  mine,  but  even  where  this  is  not  available, 
white  or  colored  chalk  or  lumber  crayon  should  be  used  to  indicate 
the  sampling  intervals  in  the  workings. 

For  tying  the  sample  sacks,  fairly  strong  twine  should  be  used. 
I  find  it  a  great  convenience,  before  going  underground,  to  cut  this 
twine  into  lengths  of  about  10  or  12  in.  and  secure  40  or  50  pieces 
together  by  an  elastic  band,  doubled  over  several  times  and  rolled 
down  to  the  middle  in  such  a  manner  that  one  piece  of  twine  can  be 
pulled  out  without  disturbing  the  others.  Fig.  5.  Some  engineers 
go  in  for  a  distinctively  colored  twine,  but  so  long  as  it  is  strong 


Fig.    5.     TWINE   CUT   IN    LENGTHS    READY   FOR    USE. 

the  color  matters  not  one  iota.  If  the  bags  are  properly  sealed,  the 
string  cannot  be  molested  without  deteection. 

Where  the  sample  is  quartered  instead  of  divided  over  a  me- 
chanical divider,  a  brush  of  some  kind  is  essential  to  remove  the  fine 
ore  remaining  on  the  canvas  and  belonging  to  the  quarters  that  are 
removed.  A  brush  is  also  essential  for  gathering  the  fine  ore  com- 
posing the  final  portion  of  the  sample,  where  the  valuable  metals  are 
contained  in  brittle  sulphides,  as  the  loss  of  the  fine  ore  may  ma- 
terially affect  the  assay  value  of  the  whole  sample.  It  is  also  used 
for  cleaning  the  canvas  sheets  and  buckets  after  the  completion  of  each 
sample. 

MECHANICAL  DIVIDERS.— There  are  various  forms  of  me- 
chanical dividers  or  samplers  in  use,  which  can  be  purchased  from 
dealers  in  assay  supplies.  Such  apparatus  is  not  only  a  great  time 
saver,  but  gives  more  accurate  results  than  the  ancient  method  of 
quartering,  which  is  still  used  to  a  great  extent  under  suitable 
conditions.  The  most  useful  form  f<j>r  mine  examination  work  is 


26 


MINE    SAMPLING   AND   VALUING 


the  so-called  Jones  sampler.  This  divides  the  sample  in  halves, 
both  portions  being  caught  in  separate  re€eptacles  placed  underneath 
the  spouts.  (Fig.  6). 


Fig.    6.     JONES    SAMPLER. 

The  theory  of  the  divider  is  that  if  the  ore  be  made  to  flow  in  an 
even  stream  over  the  top  of  the  apparatus,  it  is  automatically  divided 
by  means  of  equidistant  partitions,  which  intercept  the  stream  of  ore. 
The  quantities  passing  respectively  through  alternate  sections  com- 
bine so  that  the  sample  is  divided  into  two  equal  parts,  theoretically 
having  the  same  assay  value. 

If  the  Jones  sampler  is  not  obtainable,  a  simpler  form,  but  one 
giving  equally  accurate  results,  is  the  so-called  riffle  divider  (Fig.  7), 


Fig.    7.     RIFFLE   SAMPLER    OR    DIVIDER. 


IMPLEMENTS    EMPLOYED    IN    SAMPLING  27 

which  can  be  made  by  any  tinsmith.  This  is  a  rectangular  affair,  a 
useful  size  being  12  inches  long  by  9  or  10  in.  wide  and  about  2}^  in. 
deep.  The  partitions  may  be  5/£  to  «)4  in.  apart,  every  other  one  be- 
ing a  trough  followed  by  a  space.  This  is  set  up  in  any  convenient 
way  and  the  division  of  the  sample  is  made  by  rejecting  the  portion 
passing  through  and  retaining  the  portion  remaining  in  the  troughs. 
On  an  important  examination  in  Burma  a  few  years  ago,  such  a 
home-made  divider  was  used  successfully  in  the  preparation  of  about 
500  large  samples,  some  of  which  weighed  as  much  as  200  pounds. 


CHAPTER  IV. 

GENERAL  METHODS  OF  SAMPLING. 

T.  A.  Rickard  has  pointed  out  l  that  with  an  ore  of  the  consist- 
ency of  cheese  an  accurate  sample  can  be  taken  by  running  a  scraper 
over  it,  so  as  to  make  a  narrow  furrow  across  the  full  width  of  the 
ore.  Unfortunately  the  operation  cannot  usually  be  accomplished 
so  simply.  Nevertheless,  be  the  ore  hard  or  soft,  tough  or  brittle, 
compact  or  open  in  texture,  the  general  principle  underlying  the 
work  is  the  same.  What  is  required  is  a  true  cross-section. 

The  method  of  taking  a  sample  as  ordinarily  described  in  the 
literature  on  the  subject,  is  to  cut  a  channel  so  many  inches  wide  and 
so  many  inches  deep  across  the  orebody.  At  other  times  it  is  advised 
that  a  symmetrical  V-shaped  groove  should  be  cut  through  the  ore. 
In  my  opinion  it  is  practically  impossible  to  secure  an  accurate  result 
by  cutting  a  channel  of  a  given  depth  across  the  ore,  for  the  reason 
that  no  matter  how  carefully,  within  practical  limits,  the  face  is 
trimmed,  there  are  so  many  irregularities  in  the  face,  that  unless 
an  abnormally  large  sample  is  taken,  such  a  channel  requires  taking 
disproportionate  quantities  of  certain  parts  of  the  exposure.  As  re- 
iterated throughout  this  discussion,  the  sampler  must  use  judgment. 
Equal  quantities  as  measured  by  the  eye  are  to  be  taken. 

The  general  method  of  taking  a  sample  can  best  be  illustrated  by 
describing  a  typical  case,  such  as  a  vein  of  white  quartz,  between 
hard  and  solid  smooth  walls  of  dark-colored  rock.  Here  is  an  ore- 
body  that  to  all  intents  and  purposes  can  be  mined  clean  without  the 
admixture  of  any  country  rock,  therefore  a  sample  properly  taken 
between  the  walls  will  represent  the  value  of  the  ore  as  mined. 

The  place  where  the  sample  is  to  be  taken  is  indicated  by  mark- 
ing the  walls  with  whitewash  slightly  colored  if  necessary,  colored 
chalk  or  candle  smoke  if  nothing  else  is  available.  The  sample 
must  be  taken  from  wall  to  wall  opposite  this  point,  along  a  line  at 
right  angles  to  the  dip  of  the  orebody,  care  being  used  to  secure 
proportional  amounts  throughout  the  cut,  whatsoever  the  texture  or 
condition  of  the  rock.  The  groove  is  preferably  cut  by  means  of  a 
moil  and  hammer  and  the  chippings  caught  in  a  sampling  bucket, 
candle  box  or  other  receptacle,  and  on  completion  of  the  sample  the 
rock  chippings  are  put  carefully  into  a  sample  sack,  together  with  a 
label  marked  with  the  proper  sample  number,  after  which  the  sack 
is  securely  tied  up  and  sealed.  The  bag  containing  the  sample  should 
be  immediately  placed  in  the  leather  mail  sack,  and  if  this  is  not 
always  left  within  sight  of  the  sampler,  it  should  be  secured  by  a 
lock  to  prevent  any  possibility  of  tampering. 

llThe   Sampling  and   Estimation  of  Ore  in  a  Mine,'  page  20. 


GENERAL   METHODS   OF   SAMPLING  29 

This  in  briefest  outline,  is  the  method  to  be  pursued,  although 
there  are  a  great  many  details  connected  with  these  various  opera- 
tions on  which  not  all  engineers  are  in  agreement  and  which  on 
account  of  their  difficulty  of  application,  are  often  slighted  in  actual 
practice  by  careless  or  lazy  samplers.  These  points  will  be  dis- 
cussed in  the  subsequent  pages. 

Preparation  of  Exposure. — The  engineer  is  called  upon  to 
sample  rock  faces  of  all  kinds  from  surface  exposures  overgrown 
by  vegetation,  more  or  less  covered  with  earth  or  fungus,  to  caving 
ground  below  surface  protected  by  heavy  timber.  Aside  from  the 
physical  obstructions,  blasting  operations  may  leave  the  face  to  be 
sampled  of  irregular  shape.  In  theory,  the  width  sampled,  namely, 
the  perpendicular  distance  between  the  walls  at  right  angles  to  the 
dip,  should  be  represented  by  a  rock  face  of  that  length  and  no 
more.  In  practice  this  condition  is  rarely  met  with. 

Before  sampling  is  commenced,  the  face  to  be  sampled  must  be 
thoroughly  cleaned  and  squared-up.  Any  vegetable  matter  or  other  for- 
eign substances,  such  as  soil  or  mud,  must  be  removed,  all  irregularities 
of  surface  trimmed  off,  arched  corners  broken  down  and  in  general 
the  exposure  so  trimmed  that  it  conforms  as  nearly  as  possible  to 
the  theoretical  requirement.  In  the  very  nature  of  things  this 
requirement  is  ordinarily  impossible  of  attainment  in  practice.  I 
do  not  mean  by  this  that  it  is  unnecesary  to  square-up  or  to  clean 
a  face  to  be  sampled.  On  the  contrary,  I  am  most  insistent  that 
this  should  be  done  in  all  cases,  so  far  as  may  be  practicable,  yet 
under  the  guise  of  impracticability  the  sampler  should  not  take 
refuge  to  do  slovenly,  and,  therefore,  inaccurate  sampling.  Oc- 
casionally it  may  be  possible  to  use  explosives  to  assist  in  the 
necessary  preparation,  but  in  the  majority  of  cases  only  pick  and 
hammer  are  permissible.  With  all  the  care  possible,  the  face  pre- 
sented after  this  work  is  completed,  as  a  rule  will  be  full  of  in- 
equalities and  irregularities,  nevertheless  the  engineer  must  con- 
form to  conditions  as  he  finds  them.  Sampling  ore  is  not  as 
easy  as  sampling  cheese,  and  despite  any  irregularities  of  surface 
a  representative  sample  must  be  taken. 

In  addition  to  the  precautions  already  mentioned,  it  is  most  de- 
sirable, especially  in  damp  or  wet  mines,  to  pick  down  part  of  the 
ore,  to  present  a  clean  face  for  sampling.  In  all  mines  the  rock 
faces  become  more  or  less  covered  with  powder  smoke  and  dirt; 
this  should  be  removed  before  sampling.  In  dry  mines  it  can  often 
be  accomplished  by  merely  cleaning  the  face  with  a  stiff  brush  such 
as  a  scrub  brush  or  wire  brush.  This  film  of  dirt,  if  permitted  to 
remain,  not  only  prevents  a  satisfactory  inspection  of  the  face  to 
be  sampled,  an  important  adjunct  to  proper  sampling,  but  may 
vitiate  the  result  as  well.  Another  reason  for  picking  down  the 
face  is  to  diminish  the  likelihood  of  salting,  should  it  have  been 
prepared  for  that  purpose.  As  samples  are  usually  taken  at  regu- 
lar intervals,  interested  parties  may  measure  ahead  and  inject  gold 
into  the  rock  crevices. 


30  MINE    SAMPLING   AND   VALUING 

Where  it  is  possible  to  square  up  and  clean  a  face  it  only  re- 
mains to  take  the  sample.  Many  times,  however,  it  is  impractic- 
able to  square  a  face  to  conform  to  the  rule.  Not  only  is  this  true 
of  wide  orebodies  but  of  narrow  ones  as  well.  It  is  obvious  then 
that  other  methods  for  securing  accuracy  must  be  adopted.  The  one 
inflexible  rule  of  sampling  is  that  the  amount  of  ore  broken  into  the 
sample  must  be  proportional  to  the  width  of  the  orebody  sampled. 
For  every  inch  of  width,  there  must  be  included  in  the  sample  an 
equal  quantity,  measured  by  volume,  not  by  weight,  in  order  to 
represent  the  proportions  that  will  later  be  actually  mined.  Where 
it  is  not  feasible  to  secure  the  result  with  one  Cample,  two  or  more 
fractional  samples  must  be  taken  to  represent  one  interval.  It 
often  occurs  that  the  face  can  be  trimmed  at  right  angles  to  the 
dip  from  one  wall  part  way  to  the  other.  From  there  on,  however, 
the  face  is  arched,  and  due  to  physical  or  economic  reasons  it  can- 
not be  squared  preparatory  to  sampling.  Under  such  conditions 
one  sample  is  taken  of  the  former  portion  and  one  of  the  latter.  In 
each  case  however  the  width  sampled  is  measured  on  a  line  perpen- 
dicular to  the  dip  and  not  the  length  of  ore  face  exposed,  nor  hori- 
zontally. Cases  occur  when  a  shell  of  ore  is  left  on  one  wall  of  a 
drive  and  several  feet  of  face  must  be  sampled  to  represent  a  few 
inches  of  width.  A  few  cases  will  be  taken  as  types  and  in  a  subse- 
quent chapter  the  method  to  be  employed  pointed  out  to  serve  as  a 
guide. 

Quantity  Per  Foot  Sampled. — Under  ordinary  circumstances, 
most  engineers  in  actual  practice  cut  about  1^  Ib.  weight  of  ore  pei 
foot  of  width  sampled.  In  many  cases  it  is  less,  in  a  few  cases  it  is 
more;  in  the  latter  instance,  it  is  likely  due  to  the  softness  of  the  mate- 
rial sampled.  Engineers  vary  in  their  recommendations  of  the  amount 
to  be  taken  per  foot  of  width,  some  even  recommending  as  much  as 
4  Ib.  per  foot.  In  his  book  T.  A.  Rickard1  speaks  of  taking  samples 
of  50  to  100  Ib.  weight.  Except  in  the  case  of  the  softest  kind  of 
rock,  this  quantity  is  usually  an  economical  impossibility.  The 
amount  of  time  required  to  cut  such  a  weight  is  not  only  in  most 
cases  prohibitive,  but  the  extra  cost  of  preparing  such  large  samples 
for  assay,  with  the  means  usually  available,  not  only  adds  unneces- 
sary expense  to  an  already  expensive  operation,  but  further  weakens 
what  is  ordinarily  the  weakest  link  in  the  whole  series  of  sampling 
operations,  namely,  the  preparation  of  the  mine  sample  for  assay 
by  successive  reductions  in  size,  dividing,  and  grinding  the  final 
pulp.  Here,  as  throughout  the  sampling  operations,  the  means  at 
hand  must  be  considered.  At  a  property  having  complete  mechan- 
ical crushing  appliances,  the  size  of  the  sample  is  of  little  conse- 
quence. In  the  most  usual  case,  however,  where  only  unskilled  and 
ignorant  labor  is  available  for  crushing  by  hand,  and  where  time 
is  an  imperative  taskmaster,  small  samples  must  of  necessity  be  taken. 

The  actual  sampling  should  be  done  by  the  engineer  or  his 
trusted  assistants.  In  most  cases  an  engineer  works  alone  with  one 

^Sampling  and  Estimation   of  Ore.' 


GENERAL    METHODS   OF   SAMPLING  31 

sampling  assistant  even  on  important  examinations.  Cutting 
samples  is  hard  manual  labor  calling  for  patience  and  skill  in  using 
hammer  and  moil,  and  for  this  reason  many  engineers  delegate 
the  manual  portion  of  the  work  to  laborers.  This  is.  to  be  depre- 
cated. The  sampling  forms  the  basis  of  the  subsequent  calculations 
and  if  incorrect  the  whole  superstructure  built  on  it  as  a  foundation, 
is  like  a  house  built  on  sand.  When  intelligent  white  labor  is 
available,  it  may  be  permissible  to  follow  this  procedure,  but  in 
most  parts  of  Spanish  America,  or  countries  where  only  colored 
labor  is  available,  the  actual  cutting  of  the  sample  should  be  done 
by  the  engineer  or  his  assistants. 

Engineers  as  a  class  are  not  accustomed  to  manual  labor.  Weeks 
and  months  of  physical  inactivity  due  to  confinement  in  an  office 
make  a  man's  muscles  soft  and  the  first  few  days  of  physical  effort 
trying1  ones.  Whatever  rules  may  be  laid  down,  the  tendency  under 
such  circumstances  is  to  make  the  sample  as  small  as  consistent 
with  accuracy.  It  is  a  remarkable  thing  how  a  man's  viewpoint 
changes,  when  he  is  underground  moiling  infinitesimal  fragments 
from  an  obstinate  lump  of  hard  quartz,  that  99  times  out  of  100  has 
no  value.  Even  half  a  pound  of  ore  per  foot  of  width  sampled  has 
often  been  made  to  go,  and  without  vitiating  the  final  result. 

No  arbitrary  rule  can  be  laid  down.  Sampling  operations  must 
be  guided  by  the  engineer's  experience ;  each  proposition  must  be 
studied  separately,  even  each  individual  sample,  and  the  work 
executed  accordingly.  It  is  well  enough  to  state  a  specific  weight 
per  foot,  but  in  practice  actual  results  may  vary  greatly  from  the 
theoretical.  An  important  factor  is  the  character  of  the  material 
to  be  sampled,  hard  ore  is  more  difficult  to  cut  than  soft,  besides 
it  is  easier  to  get  a  proportional  cut  from  hard  ore  by  means  of  a 
small  sample,  except  in  cases  of  blocky  ground.  In  blocky  ground 
it  may  be  that  only  large  chunks  can  be  got  conveniently,  so  that 
the  weight  of  the  sample  is  greatly  increased.  In  soft  ground,  if 
it  be  brittle,  samples  of  any  size  can  be  obtained ;  if  it  be  clayey  or 
sticky,  or  partly  clayey  the  tendency  at  times  is  for  the  ground  to 
come  away  in  lumps,  necessitating  the  taking  of  a  similar  quantity 
of  the  remaining  ground,  and  in  that  way  making  a  large  sample. 
The  character  of  the  valuable  mineral  composing  the  ore  largely 
governs  the  weight  of  sample.  This,  as  well  as  other  points,  will 
be  taken  up  under  separate  headings. 

Interval  Sampling. — It  is  customary  in  sampling  a  mine  to  take 
samples  at  regular  intervals.  We  may  express  this  operation  by 
the  term  'interval  sampling,'  although,  curiously  enough,  I  think  it 
has  never  been  given  a  name  before.  The  object  of  sampling1  at 
regular  intervals  is  two-fold.  It  permits  of  greater  ease  in  cal- 
culating the  average  value  of  the  ore  and  is  a  preventive  against 
the  human  failing  to  sample  the  most  likely  looking  and  richest 
spots,  if  not  restricted  by  an  arbitrary  rule  of  this  character. 

One  result  of  the  optical  inspection  or  'pick  analysis'  previously 
carried  out  is  to  enable  a  better  determination  of  the  frequency 
of  the  sampling  interval.  This  varies  according  to  the  character  of 


32 


MINE   SAMPLING   AND   VALUING 


the  material  to  be  sampled.  In  high-grade  gold  ores  or  where  gold 
occurs  erratically  in  narrow  veins,  the  interval  between  samples 
may  be  not  more  than  two  feet ;  on  the  other  hand,  in  disseminated 
copper  deposits,  accurate  results  may  be  obtained  from  churn  drill 
holes  200  ft.  apart.  In  ordinary  cases  of  medium  or  low-grade  gold 
ores  a  sampling  interval  of  8  to  10  ft.  is  a  usual  one,  with  10  to  12 
ft.  in  ordinary  copper,  lead,  or  zinc  deposits.  No  rule  can  be  laid 
down,  as  the  engineer  must  use  his  own  judgment  in  each  instance 
and  be  guided  by  his  experience. 

Having  decided  on  a  sampling  interval,  which  for  purposes  of 
discussion  we  will  assume  as  10  ft.,  the  engineer  marks  the  place 
where  the  first  sample  is  to  be  taken,  in  the  manner  already  de- 
scribed, and  definitely  locates  it,  if  possible  with  reference  to  some 
survey  station,  or  otherwise  to  some  permanent  underground  work- 
ing, such  as  a  cross-cut  or  winze.  From  the  first  sample,  the  in- 
tervals are  marked  with  the  whitewash  every  10  ft.,  and  at  each 
one  of  these  places  the  entire  width  of  ore  exposed  is  sampled. 


Fig.    8.     METHOD   OF   SAMPLING   A   CROOKED   WORKING. 

Samples  should  be  taken  in  a  vertical  plane  at  right  angles  to  the  dip. 
The  dip  is  a  line  perpendicular  to  the  horizontal  line  denoting  the 
strike,  and  where  local  changes  in  the  strike  or  dip  are  encountered, 
an  adjustment  must  be  made  to  meet  the  altered  conditions. 

Where  development  workings  have  been  carried  along  one  wall 
of  an  ore  deposit,  the  almost  invariable  result  is  that  such  working 
is  more  or  less  tortuous  owing  to  the  natural  deviation  of  the  walls 
from  a  straight  line. 

Fig.  8  illustrates  what  is  by  no  means  an  uncommon  occurrence, 
a  lode  with  frequent  pinches  and  swells  and  a  big  bend.  In  the 
sketch  these  irregularities  are  somewhat  crowded  together  for  sake 
of  illustration.  The  dip  of  the  two  walls  is  not  always  the  same  and 
the  problem  is,  how  shall  the.  samples  be  taken.  This  is  shown  by 
the  sketch.  Samples  1  to  10  are  in  parallel  vertical  planes  at  right 
angles  to  the  strike,  although  the  line  of  sample  is  not  in  each  par- 
ticular case  at  right  angles  to  the  apparent  dip,  as  indicated  by  the 
walls.  From  11  onward  the  sampling  interval  is  still  the  same  ten 
feet,  measured  along  the  centre  of  the  drive,  but  due  to  its  curvature, 


GENERAL    METHODS   OF   SAMPLING  33 

the  planes  of  the  sample  cuts  are  not  parallel  either  to  each  other  or 
the  previous  lot. 

Where  a  mine  is  timbered  fresh  difficulties  are  encountered.  In 
a  case  where  a  cap  and  posts  are  used  without  lagging,  if  the 
sampling  interval  comes  opposite  a  set,  the  sample  must  be  taken 
on  one  side  or  the  other,  and  the  succeeding  samples  continued  at  the 
regular  interval  previously  marked  out.  In  closely  timbered  mines, 
where  lagging  is  used,  denoting  heavy  ground,  the  lagging  should 
be  removed  to  permit  the  samples  to  be  taken.  If  this  is  not  possible 
owing  to  the  danger  of  a  fall  of  ground,  nothing  remains  but  to 
omit  the  sample.  Cases  of  the  latter  sort  are  fairly  rare,  although 
sometimes  the  management  may  object  to  the  cost  of  removing  and 
replacing  timber,  or  for  purposes  of  deception,  overrate  the  diffi- 
culties connected  with  such  work.  The  engineer  should  in  all  cases 
satisfy  himself  on  this  point  and  if  he  believes  there  is  no  danger, 
he  should  insist  on  access  to  the  faces  to  be  sampled.  Where  one 
or  more  samples  in  succession  must  be  omitted  on  account  of  timber, 
the  question  arises  as  to  the  assay  value  and  width  to  be  assigned 
to  that  portion  of  the  working.  Either  it  must  be  taken  as  without 
value  or  included  in  the  average  of  the  level  as  determined  without 
it,  perhaps  allowing  a  certain  deduction  as  a  factor  of  safety.  The 
plan  to  be  followed  must  be  determined  by  the  engineer,  taking  into 
consideration  the  character  of  the  deposit,  its  geology,  the  mode  of 
ore  occurrence,  kind  of  mineral,  grade  of  ore,  etc.,  and  the  manner 
in  which  the  mine  is  opened.  In  marking  out  the  sampling  intervals, 
not  more  than  the  day's  work  should  be  done,  so  that  when  sampling 
is  resumed,  precautions  can  be  taken  to  prevent  being  salted. 

Fractional  Sampling. — When  an  orebody  is  more  than  four  or 
five  feet  wide,  or  where  there  are  two  or  more  bands  of  ore  of 
different  grade  or  character,  it  becomes  necessary  to  take  more  than 
one  sample  at  the  particular  interval,  and  such  operation  is  generally 
termed  'sectional  sampling.' 

This  expression  is  objectionable  to  me,  because  the  word  'section' 
as  generally  used  in  mining,  implies  the  projection  of  a  portion  or 
the  whole  of  the  mine  workings  on  to  a  vertical  plane,  whereas  in 
the  sense  of  sectional  sampling,  the  word  is  borrowed  from  civil  en- 
gineering, where  it  refers  to  a  definite  portion,  such  as  a  section  of 
road.  As  opposed  to  this  meaning,  a  sample  of  ore  is  taken  in  a 
mine,  theoretically  to  represent  a  true  cross-section  of  the  orebody. 
This  may  be  explained  as  follows :  If  a  vertical  plane  be  assumed 
to  pass  through  the  orebody  at  right  angles  to  the  strike,  at  the 
point  where  it  is  desired  to  secure  a  sample,  then  the  intersection  of 
this  vertical  plane  and  the  mine  working1  determines  the  line  along 
which  the  sample  is  to  be  taken ;  thus,  there  is  obtained  a  section  of 
the  orebody  and  to  speak  of  a  fractional  part  of  such  periphery  as  a 
section  is  technically  incorrect. 

Again,  if  more  than  one  sample  is  to  be  taken  from  any  par- 
ticular interval,  each  one  is  merely  a  fractional  part  of  the  whole, 
because  it  is  only  of  temporary  interest  and  has  been  taken  as  a 
makeshift,  to  meet  physical  conditions  beyond  the  engineer's  con- 


34 


MINE    SAMPLING   AND   VALUING 


trol ;  besides  which  the  individual  results  cannot  be  utilized,  until 
they  are  combined  and  weighted  proportionately  to  the  width  they 
represent,  in  order  to  arrive  at  the  assay  value  of  the  whole  interval. 
For  this  reason  I  consider  that  the  logical  name  for  this  operation 
is  'fractional  sampling,'  and  trust  that  the  term  may  be  adopted  to 
clear  up  the  misuse  made  of  the  expression  now  in  vogue. 

Fractional  samples  are  taken  in  wide  orebodies,  and  in  cases 
where  the  ore  occurs  in  bands  of  different  grade  or  character,  as, 
for  instance,  in  a  copper  lode  with  a  high-grade  streak,  of  say  20 
to  30%  ore  occurring  adjacent  to  low-grade  ore,  of  say  2  to  4% 
copper,  or  where  a  band  of  hard  ore  such  as  quartz,  adjoins  a  soft 
ore  such  as  decomposed  irony  or  clayey  material. 


Fig.  9.     INCLINED  OREBODY,  ONE  WALL  EXPOSED. 

The  most  usual  reason  for  taking  fractional  samples  is  because 
the  irregularity  of  the  mine  working  prevents,  even  in  compar- 
atively narrow  veins,  the  cutting  of  an  accurate  sample  in  one 
operation.  A  variety  of  cases  exists,  but  a  few  typical  samples  will 
suffice  to  illustrate  the  general  methods  to  be  employed  to  meet  the 
varying  conditions  that  may  arise. 

Case  i.     Inclined  orebody,  only  one  zvall  exposed. 

The  illustration  (Fig.  9)  represents  a  drift  following  the  foot- 
wall  of  an  orebody,  which  is  wider  than  the  drift.  The  width  of 
the  ore  is  represented  by  the  distance  F — H  perpendicular  to  the 
dip.  As  only  one  wall  is  exposed  and  that  for  perhaps  no  more 


GENERAL    METHODS   OF   SAMPLING  35 

than  a  few  inches,  and  because  nearly  all  orebodies  pinch  and  swell, 
with  the  result  that  this  face  may  be  at  a  point  of  local  variation 
from  the  general  dip  and  strike,  it  is  obviously  impossible  to  ac- 
curately determine  the  dip  from  such  a  small  surface. 

The  pick  analysis,  which  has  been  previously  carried  out,  should 
have  enabled  the  engineer  to  determine  the  average  dip  of  the 
lode  and  in  a  case  such  as  this,  the  dip  must  be  arbitrarily  assumed 
and  measurements  based  accordingly.  This  may  lead  to  a  certain 
amount  of  inaccuracy,  but  if  the  preliminary  inspection  has  been 
intelligently  done,  the  error,  if  any,  will  be  negligible.  In  any 
event,  it  is  the  best  solution  possible  under  the  circumstances,  and 
such  errors  as  occur  may  safely  be  counted  on  to  balance  one  an- 
other. 

It  is  obvious  from  the  sketch  that  the  ore  cannot  be  sampletl  on 
a  line  at  right  angles  to  the  dip,  neither  is  it  practical  to  sample  all 
around  the  exposure  in  one  cut  with  any  likelihood  of  getting  an 
accurate  sample ;  the  length  of  periphery  exposed  in  different  parts 
varies  so  greatly  to  the  width  represented,  that  it  is  not  humanly 
possible  to  do  so.  Therefore,  the  exposure  must  be  sampled  in 
fractions.  The  upper  right-hand  corner  must  be  squared  up  with 
a  pick  to  the  point  a  and  three  fractions  marked  out  by  the  points 
a  and  b. 

The  first  sample  Fi  will  be  cut  beginning  from  the  footwall  at 
'F  following  the  side  of  the  drive  to  the  point  a,  the  second  is  taken 
in  the  back  of  the  drive  from  the  point  marked  a  to  b  and  the  third 
from  b  to  W '3.  It  will  be  seen  that  sample  Fi  is  taken  along  a 
straight  line  on  the  footwall  side  of  the  drive  and,  therefore,  pro- 
portionate amounts  from  this  face  will  represent  proportionate 
amounts  of  the  width  sampled.  It  will  be  observed,  too,  that  this 
cut  includes  more  than  half  the  entire  width  of  the  ore  and  will 
affect  the  final  result,  or  average  value,  in  like  manner.  F2  is  to 
all  intents  and  purposes  a  straight  line  as  well,  but  represents  a 
lesser  width.  Fj  represents  only  a  small  fraction  of  the  entire 
width  and  the  sample  is  taken  along  a  slightly  curved  surface. 
Although  it  is  desirable,  it  may  not  be  possible  to  trim  the  corner 
as  at  a  to  take  the  sample  along  a  straight  line. 

In  most  mines  the  drives  do  not  conform  to  the  ideal  section 
shown  in  the  sketch,  Fig.  9,  and  this  last  sample,  Fj,  is  more  often 
taken  along  a  curved  line  than  along  a  straight  one.  Care  must  be 
observed  not  to  go  beyond  the  point  W$,  where  the  drive  is  nearest 
the  hanging  wall,  especially  in  a  mine  where  there  are  high-grade 
streaks,  as  otherwise  additional  amounts  of  a  band  of  ore  already 
sampled  will  be  obtained,  giving  an  inaccurate  result. 

It  is  well  to  point  out  that  under  usual  conditions  the  strata  or 
bands,  if  any  exist,  will  be  more  or  less  parallel  to  the  dip  of  the 
orebody,  and  sampling  across  the  exposed  ends  means  the  inclusion 
of  almost  the  exact  width,  wherefore  any  irregularities  of  surface 
are  not  so  likely  to  vitiate  the  accuracy  as  in  the  case  of  Fj,  where 
in  the  corner  of  the  drive  the  face  exposed  may  be  nearly  parallel 
to  the  hanging  wall,  and  if  care  is  not  taken  a  large  percentage  of 


36  MINE    SAMPLING   AND   VALUING 

the  fractional  sample  would  come  from  an  extremely  small  pro- 
portion of  the  width. 

Case  2. — Highly  inclined  vein,  one  wall  exposed,  roof 
partly  fallen. 

When  a  drift  is  untimbered  it  may  happen  that  a  block  of  ore 
will  fall  out  of  the  roof,  leaving  a  gaping  hole.  This  gives  the 
sampler  trouble.  The  indentation  is  perhaps  a  couple  of  feet  deep, 
irregular  and  in  hard  ground. 

A  general  rule  cannot  be  given ;  the  method  to  be  adopted  can 
only  be  indicated  by  a  concrete  example,  as  in  the  case  illustrated 
(Fig.  10).  An  endeavor  must  be  made  to  trim  away  the  projecting 
points  X  and  Y  presenting  three  straight  faces  F  to  a,  a  to  b  and 


l\ 

Fig.    10.     HIGHLY   INCLINED  VEIN— FACE  OF   ROOF. 

ib  to  c,  from  which  will  come  samples  Fi-F2-F^  respectively  and 
taken  in  the  way  already  described.  Where  the  character  of  the 
ground  does  not  permit  this  operation  it  may  be  necessary  to  take 
five  samples,  namely  F  to  af  a  to  x,  x  to  b,  b  to  y  and  y  to  c. 

It  can  readily  be  seen  that  the  character  of  the  ground  affects 
the  method  of  meeting  the  problem.  If  the  ground  be  firm,  it  can 
be  picked  down  or  even  blasted.  On  the  other  hand,  it  may  be  too 
hard  to  be  broken  by  pick  or  hammer  and  yet  so  loose  that  blasting 
is  out  of  the  question.  It  may  be  heavy  ground  unsafe  to  disturb 
even  with  a  pick  and  requiring  to  be  handled  very  gingerly.  Under 
ordinary  circumstances,  however,  the  condition  of  the  ground  will 
permit  of  trimming  the  face  in  such  a  way  that  an  accurate  sample 
can  be  obtained  by  following  one  of  the  methods  suggested. 

Where  the  orebody  is  vertical  Fi  will  represent  only  a  small 
portion  of  the  entire  width,  in  fact,  only  a  shell  of  ore,  and  care 
must  be  exercised  not  to  exaggerate  its  width  in  measuring,  es- 


GENERAL    METHODS   OF   SAMPLING 


37 


pecially  if  there  be  a  rich  footwall  streak.  In  this  case,  the  length 
sampled  and  consequently  the  weight  of  the  sample  exceeds  that 
of  F2  orFj  despite  the  fact  that  the  latter  two,  owing  to  their  width, 
affect  the  average  value  to  a  greater  extent.  With  an  orebody  hav- 
ing a  flat  dip  Fi  becomes  the  most  important  fraction. 

Case  3. — Irregular  roof. 

Fig.  11  represents  an  almost  similar  case  to  the  preceding  one. 
Here  we  have  a  highly  inclined  vein,  where  a  fall  of  roof  has  taken 
place  in  such  a  manner  that  the  point  a  is  almost  as  close  to  the 
footwall  as  the  lower  right-hand  corner  of  the  drive,  and  although 
a  length  of  perhaps  six  or  seven  feet  may  be  sampled,  the  actual 
width  of  vein  represented  amounts  to  perhaps  only  a  few  inches. 
In  a  case  of  this  sort  it  is  very  easy  to  make  an  error  in  measure- 
ment. 

4* 


Fig.   11.     IRREGULAR  ROOF. 

An  extreme  case  can  be  imagined  where  the  point  a  is  exactly 
the  same  distance  from  the  footwall  as  the  point  F  and  a  sample 
from  this  face  would  be  of  no  value,  as  it  represents  no  width.  Even 
where  a  small  width  is  exposed,  it  is  more  advisable  to  dig  out  the 
ore  in  the  back  of  the  ,drive  so  as  to  reach  the  footwall  at  that  point 
and  thus  permit  the  sample  being1  taken  in  the  back  of  the  drive 
instead  of  along  the  side.  The  ground  should  be  broken  down 
along  the  dotted  line  to  ai  or  to  $2,  if  possible,  and  only  one 
sample  taken  along  the  back  of  the  drive  from  ai  to  b  to  represent 
the  entire  exposure.  It  will  be  seen  that  from  b  down  to  the  floor 
represents  a  portion  of  the  orebody  already  sampled. 

A  variation  of  this  case  is  exhibited  by  the  broken  lines  V — Vi, 
V 2 — V '3  representing  the  same  occurrence  in  an  orebody  dipping  at 
a  flat  angle.  Then  the  sample  along  the  footwall  side  of  the  drive 
from  F  to  ai  would  represent  practically  the  entire  width  of  the  ex- 
posure and  a  sample  from  0,2  to  b  would  be  useless. 


38 


MINE    SAMPLING    AND    VALUING 


Case  4.     Sampling  at  a  Rise. 

If  the  sampling  interval  comes  opposite  a  rise  it  is  usually  impos- 
sible to  secure  a  sample  without  a  break  or  offset  in  the  line  of 
sampling — Fi  being  taken  in  the  rise  and  F2  in  the  back  of  the  drive. 
Often  at  a  rise  heavy  timber  is  a  necessity,  making  it  impracticable  to 
sample  at  the  proper  interval,  then  as  a  matter  of  convenience  a  sample 
is  taken  on  either  side  of  the  rise  at  half  the  regular  interval  and  proper 
allowance  made  in  the  subsequent  calculations. 

Where  a  sample  is  taken  at  a  rise,  sample  Fi  will  extend  along  its 
side  from  the  footwall  at  F  to  a  and  the  width  will  be  a — ai.  Sample 
F2  would  be  taken  across  the  back  from  a  to  b,  the  width  sampled 
being  b — bi.  If  the  orebody  were  wider  than  the  drive  then  F2  would 
extend  from  a  to  c  after  squaring  up,  or  a  third  sample  Fj  would  be 
taken  did  the  conditions  call  for  it. 


Fig.   12.     SAMPLING  AT  A  RISE. 

The  sampling  of  wide  orebodies  made  up  of  bands  of  material 
of  different  grade  and  texture  often  presents  a  troublesome  prob- 
lem, and  the  sampler  must  use  judgment  to  secure  results  that 
will  enable  a  proper  estimation  of  the  mining  possibilities.  An 
inexperienced  engineer,  one  who  did  not  understand  the  practical 
side  of  mining,  would  be  hopeless  with  a  problem  of  this  kind. 
Take,  for  instance,  a  deposit  occurring  in  slate  or  in  a  much  frac- 
tured granite  and  composed  of  bands  of  ore,  varying  in  width  from 
a  few  inches  to  as  much  as  six  feet,  the  total  width  being  say  60 
to  100  feet. 

It  may  be  accepted  as  a  fact,  that  in  a  deposit  of  this  character, 
the  separate  bands  are  not  likely  to  be  continuous  over  any  great 
length  along  the  strike,  but  that  they  will  join  and  part  company 
and  will  from  time  to  time  be  separated  by  horses  of  country  rock. 
The  sampling  must  be  done  to  afford  evidence,  whether  it  be  more 
economical  to  mine  the  entire  width  as  it  stands,  with  subsequent 


GENERAL    METHODS   OF   SAMPLING  39 

hand  picking,  both  underground  and  on  surface,  or  whether  only 
the  bands,  which  of  themselves  have  a  minable  width,  can  be 
mined  alone,  and  the  remainder  left  standing  as  pillars.  It  is  neces- 
sary to  fractional  sample  this  body,  the  size  of  the  individual  frac- 
tions depending  on  the  local  conditions.  As  a  result  of  such 
sampling,  it  may  even  be  found  that  the  more  profitable  result  can 
be  obtained  by  mining  the  orebody  by  a  milling,  shrinkage  stope, 
or  caving  method  without  any  sorting.  A  similar  problem  arises 
in  the  samples  of  underground  development  faces  in  disseminated 
copper  deposits.  The  necessity  for  experience  is  evident. 

Another  case  in  point  is  the  South  African  custom  of  sampling 
the  rich  leaders  separately,  including  a  measured  portion  of  the 
country  on  either  side.  If  two  'reefs'  are  close  enough  to  be 
worked  together,  the  intervening  country  rock  is  taken  into  the 
calculation  of  the  average  value  as  it  dilutes  the  grade  of  the  ore 
in  direct  proportion  to  its  bulk.  In  sampling  wide  bodies  of  ore, 
experience  in  that  great  mining  field  has  shown  that  fractional 
samples  of  two  feet  width  give  the  best  results.  This  width  has  not 
been  selected  on  account  of  any  irregularities  of  face  but  rather 
as  a  means  of  determining  the  distribution  of  gold  in  the  ore  and 
probably  to  counterbalance  the  unavoidable  inaccuracies  in  the 
routine  sample  of  operating  mines. 

These  few  cases  by  no  means  exhaust  the  various  conditions 
that  may  present  themselves,  but  are  deemed  sufficient  to  draw  the 
attention  of  younger  engineers  to  the  fact  that  sampling  cannot  be 
done  in  a  haphazard  manner,  nor  according  to  hard  and  fast  rules, 
but  that  the  method  adopted  must  suit  the  conditions  encountered. 

Sampling  Rises  and  Winzes. — It  is  sometimes  exceedingly  dif- 
ficult to  'sample  rises  and  winzes  especially  in  vertical  or  highly 
inclined  veins.  Probably  in  the  majority  of  cases  the  ladders  have 
been  removed,  there  is  little  or  no  timbering,  and  there  is  no  wind- 
lass handy.  The  collar  of  the  winze  may  even  be  in  such  bad 
condition  that'a  windlass  can  be  put  in  place  only  with  exceeding 
difficulty.  The  difficulties  of  inspection  or  sampling  should  not 
deter  the  engineer  from  doing  his  work  well.  In  many  instances 
these  are  increased  by  the  owners'  representatives  from  no  charitable 
motive.  Unless  an  engineer  is  willing  to  take  uninspected  winzes 
and  rises  into  his  calculations  as  worthless,  they  must  in  all  cases 
be  examined  and  sampled. 

Before  undertaking  to  sample  a  rise  or  winze,  it  must  be  care- 
fully subjected  to  pick  analysis.  The  work  must  not  be  shirked 
because  of  the  difficulty  of  moving  about  and  inspecting  the  orebody 
in  a  working  of  this  kind,  when  suspended  in  a  bucket,  boatswain's 
chair,  or  when  balancing  oneself  on  shaky  planks  or  poles  laid  across 
uneven  timber.  The  optical  inspection  is  essential.  It  enables  the 
engineer  to  determine  such  conditions  as,  whether  the  full  width 
of  the  orebody  is  exposed,  whether  the  working  follows  one  of  the 
walls,  or  is  tortuous,  and  affords  evidence  as  to  the  exposure  of 
any  known  high-grade  streak,  as  well  as  any  change  due  to 
geological  causes,  such  as  the  occurrence  of  the  values  in  floors. 


40  MINE    SAMPLING   AND   VALUING 

Rises  and  winzes  are  usually  made  for  one  of  three  reasons, 
namely:  (1)  Exploration;  (2)  ventilation;  (3)  blocking  out  the  ore- 
body  to  facilitate  subsequent  mining  operations.  If  the  purpose  is 
for  exploration,  then  in  all  probability  the  rise  or  winze  will  follow 
the  orebody  in  all  its  twists  and  turns,  and  the  tendency  will  be, 
unless  the  vein  is  very  wide,  to  expose  its  full  width. 

When  either  the  second  or  third  object  is  to  be  attained,  it 
is  likely  that  the  working  will  be  more  or  less  straight,  in  which  case 
the  orebody  may  be  left  partly  or  altogether  in  one  of  the  walls. 
The  sampling  must  be  done  to  meet  whichever  one  of  these  con- 
ditions is  encountered,  and  the  interpretation  of  the  results  is  ef- 
fected in  a  like  manner.  When  the  orebody  is  wider  than  the  rise 
or  winze  and  only  a  portion  of  its  total  width  is  exposed,  unless 
carried  along  one  of  the  walls,  the  working  may  cross  and  even 
recross  a  given  run  of  valuable  ore.  In  a  case  of  this  kind  sections 
of  such  workings  should  be  carefully  platted  from  actual  surveys 
and  the  sampling  results  carefully  interpreted.  On  the  other 
hand,  if  the  whole  width  of  the  orebody  is  exposed,  samples  taken 
at  regular  intervals  will  furnish  resultS4that  may  be  used  for  cal- 
culating the  general  averag'es  required  for  arriving  at  the  estimates 
of  tonnage. 

When  only  a  portion  of  the  orebody  is  exposed,  the  sampling 
results  cannot  be  used  for  this  purpose,  but  merely  to  serve  as  a 
guide  and  as  some  evidence  of  the  persistence  of  values  from  one 
level  to  another,  and  this  must  be  kept  in  view  while  the  sampling 
is  being  done. 

When  only  one  wall  is  exposed,  the  utmost  caution  must  be 
observed  in  case  a  high-grade  streak  is  included  in  the  portion  of  the 
orebody  that  can  be  sampled.  When  the  ore  passes  in  and  out  of 
the  winze  or  rise,  because  either  the  vein  or  the  working  is  tortuous, 
such  ore  as  is  exposed  should  be  sampled  and  the  width  noted,  and 
the  results  used  as  a  guide  in  sizing  up  the  deposit.  When  the  wall 
rock  is  sampled,  it  must  not  be  included  in  the  same  sample  as  the 
ore,  but  must  be  given  a  separate  number  and  assayed  separately. 

In  some  orebodies  there  is  a  tendency  for  the  valuable  ore  to 
occur  in  floors  or  flat-pitching  shoots,  and  the  vertical  development 
openings  afford  information  of  the  most  useful  character.  In 
limestone  deposits  the  occurrence  of  ore  along  floors  or  flat-lying 
beds  is  quite  common  and  the  evidence  afforded  by  winzes  or  rises 
in  this  type  of  deposit  is  of  the  utmost  importance.  In  cases  of  this 
kind  sufficient  information  may  even  be  obtained  by  optical  in- 
spection to  form  an  adverse  opinion  and  thus  obviate  the  expense 
of  sampling  the  mine. 

Particular  attention  should  be  paid  to  winzes  below  the  bottom 
level.  When  developments  in  the  bottom  are  bad,  it  is  a  common 
trick  to  allow  winzes  to  fill  with  water,  or  they  may  even  be  in- 
tentionally filled  with  waste  to  prevent  inspection.  The  engineer 
cannot  be  too  cautious  on  such  an  occasion.  A  mine  with  a  bad 
bottom  is  usually  a  bad  bargain.  Once  eliminate  the  possibilities 
of  extension  in  depth,  then  the  mine  has  no  prospective  value,  and 


GENERAL    METHODS   OF   SAMPLING  41 

if  you  eliminate  the  prospective  value  of  a  mine,  the  chances  of  big 
profit  and  long  life  are  gone — factors  which  play  an  important  part 
in  mining  operations  and  which  cannot  be  overlooked. 

Sampling  Cross-cuts. — Occasionally  a  mine  is  opened  up  by 
cross-cuts  only,  the  drifts  being  carried  in  the  country  because 
greater  speed  can  be  made.  The  reliance  that  can  be  placed  on  the 
results  from  development  openings  of  this  kind  depends  on  their 
frequency,  at  the  same  time  taking  into  consideration  the  character 
of  the  ore  deposit.  Under  no  circumstances  is  the  information  af- 
forded as  satisfactory  or  reliable  as  is  the  case  where  the  drifts  have 
been  carried  in  the  orebody.  Particularly  is  this  true  with  gold 
ores,  but  even  with  base  metals  where  there  is  any  tendency  for 
a  segregation  of  the  mineral,  care  must  be  exercised  in  interpret- 
ing results.  The  character  of  the  mineralization,  therefore,  to  a 
large  extent  governs  the  manner  of  sampling,  not  only  where  an 
orebody  is  opened  only  by  cross-cuts,  but  also  in  the  sampling  of 
any  cross-cuts  whatsoever. 

For  purposes  of  discussion,  assume  a  case  where  the  full  width 
of  the  ore  is  exposed.  The  ore  may  be  sampled  along  the  sides, 
back  or  floor.  Sampling  the  floor  is  usually  out  of  the  question  on 
account  of  its  greater  inconvenience.  The  most  common  plan, 
especially  if  the  drift  is  in  the  orebody,  is  to  sample  across  the  back, 
because  the  full  width  can  be  sampled  without  a  break  in  the  cut. 
When  sampling  the  sides,  a  break  occurs  at  the  junction  with  the 
drift,  and  the  sampling  must  be  finished  across  the  back  of  the 
drift.  The  method  to  be  followed  is  governed  by  the  sampler's 
experience. 

In  sampling  deposits  of  the  massive  type,  such  as  disseminated 
copper  deposits,  or.  bedded  lead-zinc  deposits,  the  best  results  are 
obtained  by  dividing  the  cross-cut  into  5-foot  intervals  and  making 
a  continuous  cut  along  the  back  and  two  sides  and  combining  them 
for  one  sample.  If  there  is  a  good  bunch  of  ore  in  one  place,  it  is 
easy  enough  to  get  a  false  result  by  taking  more  than  its  due  pro- 
portion. 

One  must  be  careful  to  get  as  nearly  as  possible  the  same  weight 
of  ore  from  each  of  the  three  cuts.  In  fact,  the  precautions  already 
set  forth  at  length  elsewhere  are  to  be  observed,  and  if  advisable 
each  fraction  may  be  assayed  separately  and  the  results  averaged 
in  the  usual  way. 

I  have  seen  a  cross-cut  sampled  in  a  sort  of  spiral,  namely,  from 
the  floor  on  one  side,  up  the  side,  across  the  back  and  down  on  the 
other  side,  the  cut  advancing  and  finishing  5  ft.  or  10  ft.  in  advance 
of  where  it  started.  The  only  thing  that  can  be  said  of  that  method 
is,  it  is  different  from  the  generally  accepted  method,  it  is  hard  to 
see  any  increased  accuracy  resulting  therefrom,  while  on  the  other 
hand,  it  is  more  difficult  to  cut. 

In  inclined  orebodies,  and  they  include  the  vast  majority  with 
which  engineers  have  anything  to  do,  sampling  across  the  back 
or  sides  in  a  continuous  groove- involves  taking  horizontal  measure- 
ments, unless  by  means  of  a  string  the  angle  of  inclination  is  laid 


42  MINE    SAMPLING   AND   VALUING 

off  and  the  true  width  of  each  fraction  measured  as  it  is  taken.  If 
the  horizontal  measurement  be  taken,  the  true  width  should  be 
calculated  and  set  down  in  the  note  book  for  use  in  the  subsequent 
calculations  and  assay  plan.  This  is  determined  by  the  following 
formula : 

True  width  =  Horizontal  measurement  x  angle  of  dip. 

Another  way  is  to  figure  out  beforehand  the  horizontal  distance 
corresponding  to  the  true  width  of  the  fraction  it  is  desired  to  take 
and  to  lay  off  the  calculated  length  in  the  cross-cut  before  sampling, 

thus  let  h  =  the  horizontal  width, 

w  =  the  true  width  of  the  fraction, 
a  —  angle  of  dip. 


then  h  = 


In  orebodies  where  there  are  strata  or  seams,  more  or  less  parallel 
to  the  dip  of  the  orebody,  fractional  samples  are  sometimes  taken  across 
the  true  width,  the  successive  fractions  making  a  staggered  line,  like  a 
flight  of  stairs.  This  is  not  to  be  recommended.  Wherever  possible 
samples  should  be  cut  from  a  continuous  groove.  Seams  may  contain 
ore  of  high  value  and  by  staggering  the  samples  there  is  greater  danger 
of  getting  an  excessive  amount  of  the  high  grade. 

The  calculation  of  tonnage  in  an  orebody  opened  by  cross-cuts  alone 
is  a  proceeding  that  often  is  attended  with  considerable  risk*.  Even 
where  there  is  no  attempt  at  deception  on  the  part  of  the  owners,  cross- 
cuts are  often  put  in  at  likely  looking  places  and  the  resultant  calcula- 
tions are  then  in  excess  of  the  breaking  value  of  the  ore.  Horses  of 
rock  and  bunches  of  low-grade  ore  have  a  habit  of  making  in  most 
unexpected  ways. 

When  the  orebody  must  be  studied  from  cross-cuts  alone  a  certain 
amount  of  misconception  may  arise  on  account  of  parting  planes  and 
seams  dipping  in  misleading  directions,  so  that  the  samples  taken  are 
not  gauged  according  to  the  real  dip  of  the  orebody.  Especially  is  this 
true  where  the  dip  of  an  orebody  changes  or  reverses.  A  single  cross- 
cut in  what  appears  to  be  a  wide  orebody  may  be  on  a  branch  vein  and 
a  few  feet  on  either  side  of  the  so-called  cross-cut  the  valuable  ore  may 
terminate.  When  an  orebody  has  been  drifted  on,  cross-cuts  give  valu- 
able information ;  besides,  corroboratory  evidence  can  be  obtained  by 
drill  holes  from  the  sides  of  the  drifts.  In  the  massive  type  of  deposit, 
care  must  be  exercised  in  calculating  tonnages  and  values  near  the 
edges  of  the  deposit.  It  is  particularly  in  cases  of  this  sort  that  blocks 
of  barren  ground  may  be  included. 

The  principle  involved  is  that  cross-cuts  alone  do  not  prove  the  con- 
tinuity of  the  orebody  in  the  same  manner  that  drifts  do  and  therefore 
cannot  be  relied  on  to  the  same  extent.  The  distance  between  cross- 
cuts is  an  important  factor  and  the  greater  their  frequency  the  more 
reliance  can  be  placed  on  the  results  obtained.  In  any  case  caution  is 
necessary. 


GENERAL    METHODS   OF   SAMPLING  43 

Measurement  of  Width  Sampled. — It  has  been  stated  that  the 
widths  sampled  should  be  measured  at  right  angles  to  the  dip  of  the 
lode.  Referring  to  Fig.  9,  it  will  be  seen  that  there  is  only  one  fixed 
reference  point,  namely  the  footwall  where  it  is  exposed  at  F.  Sam- 
ple Fi  was  cut  along  the  side  from  F  to  a  and  the  width  actually  sam- 
pled is  represented  by  the  distance  F — Wi. 

The  only  accurate  method  of  measuring  that  width,  is  to  stretch  a 
string  from  a  parallel  to  the  dip  and  measure  the  distance  between  F 
and  W I  by  means  of  a  steel  tape.  The  end  of  the  string  at  a  may  be 
held  by  the  sampler's  assistant  or  secured  with  a  nail — the  lower  end 
against  the  opposite  side  of  the  drive  may  be  held  in  place  by  the  hand 
or  by  winding  the  string  around  a  hammer,  pick,  or  other  tool. 

The  string  a-ai  can  be  accurately  lined  up  by  going  off  a  few  feet 
and  gauging  the  angle  of  the  string,  which  is  illuminated  by  a  candle 
held  behind  it.  The  width  of  Fs  can  usually  be  determined  by  using 
another  string  or  by  lining  up  a  piece  of  steel.  Finally  the  width  of 
Fj  is  taken  by  stretching  the  tape  to  the  farthermost  corner  and  sub- 
tracting the  width  of  Fs.  By  this  method  there  is  less  liability  of 
error  than  if  the  measurement  were  attempted  direct.  This  method  is 
accurate  beyond  question  and  immediately  gives  an  absolute  result,  re- 
gardless of  any  twists,  turns  or  rolls  in  the  formation  and  there  is  no 
overlapping  of  measurements. 

Another  way  that  is  used  by  some  engineers  is  to  measure  the  widths 
by  means  of  sticks.  This  is  not  only  cumbersome  and  apt  to  give 
inaccurate  results  in  a  narrow  mine  working,  perhaps  necessitating 
its  rejection,  but  the  proper  gauging  of  the  width  is  difficult,  and 
therefore  there  is  more  liability  to  error.  In  mine  sampling,  every 
safeguard  it  is  possible  to  employ  to  avoid  mistakes  should  be  used; 
as  there  are  enough  uncertain  factors  in  the  work  that  cannot  be 
provided  against  and  the  engineer  should  not  multiply  them. 

Some  engineers  take  the  horizontal  measurement.  In  wide  veins 
or  in  a  case  such  as  Fig.  9  (p.  34),  it  is  not  only  impracticable  but 
likewise  apt  to  be  inaccurate,  although  in  veins  narrower  than  the 
drive  it  may  be  applied.  The  theory  underlying  horizontal  measure- 
ments is  that  all  the  measurements  are  proportional  in  the  same  degree 
to  the  true  width,  but  the  method  gives  incorrect  results,  where  the 
dip  of  the  orebody  changes  along  its  strike  or  dip,  as  the  following 
illustration  will  demonstrate. 

In  calculating  the  volume  of  a  block  of  ground  for  tonnage  pur- 
poses where  the  true  width  of  the  ore  has  been  measured,  the  area 
is  obtained  by  taking  half  the  sum  of  the  two  widths  on  either  side 
of  the  block,  which  is  multiplied  by  the  distance  between  them  as 
measured  along  the  plane  of  the  orebody. 

Where  horizontal  measurements  are  used  a  cross-section  shows  a 
rhomboid  instead  of  a  rectangle  and  to  get  the  area  half  the  sum  of 
the  two  horizontal  measurements  is  multiplied  by  the  perpendicular 
distance  between.  Where  the  orebody  rolls,  the  volume  thus  obtained 
is  incorrect,  because  this  method  is  based  on  the  theory  of  proportional 


44  MINE    SAMPLING   AND   VALUING 

length  between  vertical  and  hypotenuse.  When  the  orebody  takes 
a  roll,  the  length  of  the  hypotenuse  may  be  considerably  augmented. 
Horizontal  measurements  are  no  doubt  easier  to  take,  but  they  do 
not  give  results  as  accurate  as  those  obtained  by  measuring  the  true 
width  direct.  It  may  be  claimed  for  horizontal  measurements  that 
the  results  are  on  the  safe  side,  nevertheless,  the  engineer  must  guard 
against  inaccuracies  of  any  kind,  for,  as  already  emphasized,  if  it  is 
desired  to  use  a  factor  of  safety,  it  must  not  be  left  to  chance. 


CHAPTER  V. 

PRECAUTIONS  NECESSARY  IN  SAMPLING. 

The  foregoing  paragraphs  have  dealt  with  the  principles  under- 
lying sampling  operations,  and  a  few  of  the  problems  that  may  be 
encountered  in  mining  operations  have  been  illustrated.  This  chapter 
will  deal  largely  with  certain  mechanical  operations  connected  with 
the  handling  of  the  sample  underground.  It  has  been  pointed  out  that 
the  place  where  the  sample  is  to  be  taken  should  be  marked  by  white- 
wash or  chalk ;  that  the  cut  should  be  a  straight  one ;  should  be  repre- 
sentative of  the  width  sampled,  and  should  be  caught  in  a  duck  bucket 
or  other  similar  receptacle. 

Sampling  a  homogeneous  material  or  one  where  the  valuable  ore  is 
evenly  distributed  through  the  gangue,  does  not  require  the  same 
amount  of  care  as  in  the  case  where  it  is  spotted.  For  example,  in 
sampling  an  iron  ore  deposit  or  low-grade  copper  ore,  an  accurate  sam- 
ple can  be  secured  in  most  instances  by  smashing  down  the  ore  with  a 
mining  pick  into  a  large  canvas  sheet ,  although  such  a  procedure  is 
not  recommended. 

Low-grade  copper  and  iron  ore  deposits  are  bought  and  sold  on  the 
results  of  drill-hole  samples,  and  from  a  sampling  standpoint  are  in  a 
class  by  themselves.  Under  ordinary  circumstances  the  difficulties  of 
sampling  that  present  themselves  to  an  engineer  are  largely  due  to  the 
brittleness  of  the  valuable  mineral  in  a  harder  gangue,  and  also  to  the 
occurrence  of  erratic  values  in  the  ore.  For  this  reason  the  use  of 
the  sampling  bucket  gives  more  reliable  results  than  the  canvas,  and 
largely  represents  the  difference  between  sampling  for  valuation  per  se 
by  an  independent  engineer,  and  the  methods  employed  in  taking  the 
routine  samples  by  the  mine  staff  in  going  mines. 

Where  the  ore  contains  sulphides,  the  blow  of  the  hammer  on  the 
moil  jars  the  ground,  and  has  a  tendency  to  shake  down  the  brittle 
mineral.  The  larger  the  area  exposed  to  receive  the  falling  particles 
of  mineral,  the  greater  the  likelihood  of  contaminating  the  sample,  with 
what  is  usually  the  richest  constituent  of  the  orebody.  Many  people 
state  that  a  mine  cannot  be  sampled  to  show  its  stoping  value.  If 
sampling  is  properly  done,  there  is  no  reason  why  it  should  show  a 
higher -value  than  can  be  mined,  barring  the  admixture  of  the  wall  rock 
with  the  ore,  for  which  allowance  should  be  made. 

In  operating  mines,  experience  shows  that  the  daily  mine  samples 
must  be  reduced  in  value  by  10  to  15%  to  show  the  mill  head  value  or 
assay  value  at  the  face.  This,  in  my  opinion,  is  due  to  the  necessity 
of  hurry  in  taking  daily  mine  samples,  for  which  reason  less  care  is 
possible.  Such  samples  come  from  the  development  faces  and  stopes, 
and  while  the  sampler  is  at  work,  the  miners  are  compelled  to  be  idle, 
and  because  assay  results  are  required  the  same  day,  speed  is  an  es- 


46  MINE    SAMPLING   AND   VALUING 

sential  and  this  error  due  to  the  method  employed  is  allowed  for  by  a 
factor  of  correction.  While  the  factor  employed  may  be  incorrect  in 
individual  instances,  these  errors  are  presumed  to  balance  one  another. 

The  usual  method  of  taking  daily  samples,  is  to  lay  a  large  canvas 
on  the  ground  on  which  the  sampler  steps,  and  by  means  of  able-bodied 
blows  with  a  pick,  a  portion  of  the  ore  in  the  face  is  broken  down  into 
the  canvas.  Inevitably  by  this  method  chunks  often  fall  from  the  face 
into  the  canvas,  become  mixed  with  what  is  already  there,  and  make  an 
inaccurate  sample.  Above  all,  however,  the  chief  error  originates  from 
the  brittle  and  valuable  sulphides  being  jarred  down  into  the  canvas 
from  a  considerable  area  of  the  face  by  the  force  of  the  blow. 

In  mine  examination  work,  the  sample  is  carefully  cut  by  moil  and 
hammer,  and  as  much  time  as  necessary  to  secure  an  accurate  result 
is  taken  by  the  engineer.  In  many  places,  not  over  ten  or  twelve  sam- 
ples per  day  can  be  gotten  by  a  good  sampler  with  one  or  two  helpers. 
In  the  Broken  Hill  South  Blocks  mine,  the  ore  was  so  hard  that  a 
sampler  with  two  skillful  miners  working  double-handed  was  able  to 
secure  only  five  or  six  samples  a  day.  This  is,  of  course,  rare.  The 
result  is  that  in  that  district  daily  face  samples  are  not  regularly  taken 
in  the  mines. 

In  cutting  a  sample  in  mine  valuation  work,  if  a  piece  of  ore  flies 
beyond  the  bucket,  it  should  not  be  picked  up  from  the  floor,  but  an- 
other piece  chipped  from  the  face.  Picking  pieces  up  from  the  floor  is 
likely  to  lead  to  error  and  danger  of  salting.  If  too  large  a  quantity 
accidentally  comes  away,  and  falls  into  the  bucket,  nine  times  out  of 
ten  the  sample  should  be  thrown  away  and  a  new  start  made.  This 
may  involve  considerable  additional  work,  but  should  not  be  neglected 
except  in  the  sole  case  where  the  ore  has  come  away  in  a  single  chunk, 
and  can  be  picked  out,  but  even  this  is  bad  practice. 

Care  should  be  taken  not  to  include  too  much  of  the  softer  seams,  if 
there  are  any.  These,  too,  may  represent  the  richer  portion  of  the  ore. 
The  greatest  caution  must  be  used  in  sampling  high-grade  streaks,  and 
if  possible  they  should  be  sampled  separately.  Erratic  high  assays 
often  represent  carelessness  in  sampling.  Hard  ground  being  more 
difficult  t£>  cut,  offers  a  temptation  to  the  sampler  to  take  a  smaller 
quantity  than  required  to  represent  it,  and  great  self-restraint  is  often- 
times necessary  on  the  sampler's  part  to  keep  him  from  moving  ahead 
before  he  has  secured  a  proper  quantity  from  hard  ground,  urged  on 
as  he  may  be  by  tired  muscles. 

A  quantity  of  the  wall  rock  should  be  included  in  every  sample,  not 
only  to  make  sure  that  the  limit  of  valuable  ore  has  been  reached,  but 
also  because  when  stoping  the  ore  is  bound  to  become  diluted  to  a  cer- 
tain extent  by  falls  of  the  country  into  the  ore. 

In  orebodies  where  the  ore  occurs  in  strata  or  streaks  of  different 
texture  and  hardness,  a  condition  presents  itself  that  often  makes  ac- 
curate sampling  difficult.  In  the  preliminary  work  preceding  the 
active  sampling  operations,  if  an  assay  plant  or  other  means  of  testing 
is  present,  a  few  samples  may  perhaps  be  taken  to  advantage  for  the 
purpose  of  determining  the  distribution  of  the  valuable  minerals.  It 


PRECAUTIONS    NECESSARY    IN    SAMPLING  47 

may  be  found  that  a  certain  kind  of  quartz  carries  most  of  the  gold, 
in  which  case  it  emphasizes  the  desirability  of  sampling  such  quartz 
separately. 

In  the  *L'  mine  the  vein  is  composed  of  hard  quartz,  in  places 
honeycombed  and  like  gossan,  due  to  the  oxidation  of  pyrite,  a  yellow- 
ish clay  and  manganese  oxide,  which  occur  in  a  more  or  less  banded 
structure.  Careful  tests  failed  to  point  out  any  one  of  these  as  carry- 
ing an  abnormal  amount  of  precious  metals  as  compared  to  the  others. 
Had  there  been  a  compact  band  of  quartz  followed  by  a  band  of  soft 
ore,  the  work  would  have  resolved  itself  into  taking  a  sample  from 
each.  As  a  matter  of  fact,  however,  no  regularity  exists.  Often- 
times a  narrow  seam  of  quartz  occurs  embedded  as  it  were  in  the 
clay,  which,  wlien  struck,  would  retreat  into  the  yielding  material. 
So  marked  was  this  that  in  instances  it  was  necessary  to  complete 
the  remainder  of  the  cut  and  then  dig  out  the  quartz  and  break  off 
the  desired  amount  on  the  floor  of  the  working.  In  a  case  of  this 
sort,  however  desirable  it  may  be,  it  is  manifestly  out  of  the  question 
to  sample  this  small  seam  of  hard  material  separately  from  the  rest. 
It  did  not  assay  very  differently  from  the  rest,  and  while  an  error  may 
creep  in,  it  is  reduced  to  such  a  small  percentage  that  it  does  not 
appreciably  vitiate  the  result. 

In  this  mine  great  liberties  were  taken  in  the  sampling  by  various 
engineers  who  had  previously  examined  it,  yet  the  average  values  ob- 
tained did  not  materially  differ  one  from  the  other.  While  this  is 
pointed  out,  it  is  by  no  means  intended  that  lax  methods  may  be  em- 
ployed. The  exception  but  proves  the  rule.  In  sampling  this  vein 
in  places  the  quartz  was  so  hard  that  it  would  take  an  hour  to  moil  12 
to  15  in.,  whereas  the  clayey  and  manganiferous  material  came  away 
so  easily  that  only  the  lightest  blows  from  a  prospecting  pick  were  per- 
missible to  prevent  knocking  down  excessive  quantities.  It  is  in 
material  of  this  character  that  a  prospecting  pick  may  be  used  in 
preference  to  a  moil  and  hammer.  This,  too,  is  one  of  the  cases  where 
an  extreme  condition  exists,  and  where  in  most  cases  caution  is  re- 
quired to  prevent  inaccurate  sampling  by  taking  too  large  a  propor- 
tion of  one  or  the  other  kind  of  ore.  At  times  the  soft  material 
would  come  away  in  a  chunk ;  at  others  too  large  a  piece  of  the  hard 
ore. 

In  orebodies  of  the  type  of  the  Broken  Hill  deposits,  the  valuable 
minerals,  i.  e.,  the  comparatively  soft  and  brittle  lead  and  zinc  sul- 
phides (galena  and  blende)  occur  in  a  gangue  of  garnet  rock.  Gar- 
net rock  is  one  of  the  hardest  rocks  that  the  sampler  ever  has  to  en- 
counter and  under  the  moil  is  most  refractory,  even  when  using  a 
double-hand  hammer.  Here  we  have  two  extremes,  and  unless  great 
care  is  used,  the  samples  will  all  run  high  on  account  of  the  excessive 
proportion  of  the  valuable  friable  mineral,  which  is  not  only  easier  to 
break  down,  but  shakes  into  the  sample  from  above  at  every  blow  of 
the  hammer. 

Bodies  of  solid  pyritic  material  would  seem  to  offer  the  simplest 
sort  of  problem  for  sampling,  yet  even  in  this  class  of  deposit  care 


48  MINE    SAMPLING   AND   VALUING 

must  be  exercised  against  unconscious  salting,  where  the  iron  pyrite 
contains  copper  pyrite.  As  usual  in  this  class  of  deposit,  amount  of 
copper  is  not  evenly  distributed  throughout  the  mass,  but  it  occurs 
segregated  in  bunches,  streaks,  nodules  and  lenses.  As  is  well  known, 
one  of  the  simplest  means  of  differentiating  between  iron  and  copper 
pyrite  by  optical  inspection  is  the  test  for  hardness,  ordinary  pyrite 
being  unscratchable  with  the  knife  blade,  whereas  copper  pyrite  is 
easily  scratched.  Hence  in  sampling  in  this  class  of  deposit,  care 
must  be  exercised,  because  on  account  of  the  softness  of  the  copper 
pyrite,  excessive  amounts  may  be  taken,  thereby  unduly  raising  the 
copper  contents  of  the  sample. 

Another  case  which  might  be  pointed  out  is  the  occurrence  of  low- 
grade  silicious  streaks  or  horses  in  the  midst  of  orebodies  of  this  class. 
These  silicious  horses  often  contain  good  percentages  of  copper  and 
therefore  constitute  part  of  the  orebody.  And  even  in  the  case  where 
from  an  optical  inspection,  it  would  appear  that  such  silicious  ma- 
terial does  not  contain  sufficient  copper  to  be  classed  as  ore,  it 
should  in  most  cases  be  sampled,  because  when  correlating  the  results 
of  the  sampling  operations  and  deciding  on  a  method  of  mining,  it 
may  be  found  more  economical  or  even  essential  to  stope  these  silicious 
horses  with  the  remainder  of  the  orebody.  However,  each  case  must 
be  judged  on  its  merits. 

In  pyrite  masses  there  is  usually  a  tendency  for  a  segregation  of 
the  copper,  with  the  result  that  often  there  is  a  rich  and  usually  per- 
sistent streak  along  one  of  the  walls.  Sampling,  therefore,  must  be 
carried  on  with  a  view  of  determining  the  value  of  this  higher  grade 
streak  separately  from  the  larger  bulk  of  lower  grade  material  adjoin- 
ing it,  because  it  may  be  found  that  when  the  final  calculations  are 
made  the  economic  conditions  may  only  permit  mining  this  high-grade 
streak. 

In  sampling  ore  that  is  narrower  than  stoping  width  only  the  ore 
should  be  sampled.  It  is  bad  practice  to  include  in  the  sample  barren 
country  rock  to  the  full  stoping  width.  Not  only  is  it  useless  labor, 
but  it  hides  the  real  width  and  value  of  the  orebody  at  that  point  and 
possibly  prevents  a  proper  interpretation  of  results.  In  making  the 
calculation  for  tonnages  the  narrow  widths  can  be  calculated  to  stoping 
width  by  reducing  the  assay  in  the  proper  proportion. 

Where  a  streak  of  high-grade  ore  is  opened  in  a  working  in 
which  the  full  width  of  the  orebody  is  not  exposed,  the  assay  of  the 
sample  at  this  point  is  too  high,  and  therefore  the  assay  value  of  the 
interval  should  be  reduced.  That  ore  of  high  value  may  occur  erratic- 
ally is  well  known,  and  in  gold  mines,  where  it  does  so  occur,  it  is 
customary  to  reduce  the  high  assays  to  the  general  average  of  their 
neighbors.  In  wolfram  or  mixed  wolfram  and  tin  deposits,  the  min- 
eral is  apt  to  occur  in  rich  pockets  or  bunches  surrounded  by  low- 
grade  or  even  barren  gangue.  In  such  cases  the  erratic  results  must 
be  averaged  with  the  others  to  arrive  at  the  stoping  value  of  the  ore. 
In  some  gold  deposits  high-grade  streaks  occur  in  conjunction  with 
low-grade  ore,  the  high  grade  containing  visible  free  gold.  It  is  per- 


PRECAUTIONS    NECESSARY    IN    SAMPLING  49 

haps  needless  to  state  that  each  band  must  be  sampled  separately  and 
a  great  deal  of  judgment  used  in  bringing  the  high-grade  results  into 
the  calculations. 

In  the  replacement  type  of  gold  deposits,  where  the  precious  metal 
occurs  along  cracks  or  fractures  in  the  rocks,  caution  must  be  observed 
that  a  sample  does  not  follow  one  of  these  lines  of  enrichment.  The 
mineral  may  occur  in  parallel  lines  of  fracturing  within  the  orebody 
itself,  which  fact  should  be  discovered  by  pick  analysis,  and  the 
sampling  done  to  meet  that -condition.  In  some  deposits,  especially 
in  limestone,  the  ore  is  deposited  in  floors,  or  flat  'makes,'  of  which 
there  may  be  several  separated  by  unprofitable  material.  If  this  be 
discovered  in  time,  the  whole  expense  of  sampling  may  possibly  be 
avoided,  although  some  of  the  richest  mines  known  have  deposits 
replacing  soluble  limestone  beds. 

If  an  engineer  is  conscientious  he  will  endeavor  to  secure  as  much 
information  as  possible  regarding  the  assay  value  and  quantity  of  ore 
available.  It  is  bad  practice  not  to  get  information  as  to  the  charac- 
ter of  the  ore  left  in  the  sides  of  drifts,  etc.,  where  the  orebody  is 
wider  than  the  workings.  Where  only  an  occasional  bulge  occurs  it 
may  not  be  worth  while,  but  under  ordinary  circumstances  not  only 
should  the  exposed  faces  be  sampled  but  horizontal  drill  holes  put  in 
the  sides  by  steel  and  hammer  and  the  drippings  assayed.  It  is  not 
only  desirable  on  the  score  of  the  increased  tonnage  available,  but  also 
because  in  mining,  this  portion  of  the  orebody  will  be  broken  down 
with  the  remainder  and,  if  low  grade,  may  turn  a  profit  into  loss.  In 
case  of  wide  veins  where  there  are  insufficient  cross-cuts,  diamond 
drills  can  often  be  employed  to  advantage,  though  not  often  available. 


CHAPTER  VI. 

GEOLOGICAL  FACTORS  IN  SAMPLING  AND  VALUATION. 

In  this  chapter  it  is  proposed  to  call  attention  to  some  of  the 
geological  factors  appertaining  to  mine  sampling  and  valuation  not 
discussed  elsewhere.  That  the  geology  of  an  ore  deposit  has  an 
important  bearing  on  the  sampling  operations  is  evidenced  by  the 
necessity  for  pick  analysis,  which  has  already  been  elucidated.  Be- 
sides this  it  has  a  serious  bearing  on  the  valuation  of  the  ore  re- 
serves, because  a  knowledge  of  the  geology  of  the  deposit  is  essential 
in  forming  a  definite  opinion.  Its  importance  can  therefore  be 
appreciated.  A  mistake  often  made  by  engineers  in  mine  examination 
work  is  that  valuable  time  is  lost  making  a  more  or  less  detailed  study 
of  the  general  geology  of  the  district,  that  could  be  used  to  better 
advantage  underground. 

One  thing  should  be  remembered,  namely,  that  the  mining  en- 
gineer is  rarely  an  expert  geologist  and  the  geologist  even  more  rarely 
a  proficient  mining  engineer.  The  character  of  the  work  of  each 
is  different,  as  is  the  object  to  be  attained.  The  work  of  the 
geologist  deals  not  at  all  with  the  commercial  aspect,  but  with 
the  results  of  cause  and  effect;  he  traces  out  the  geological 
phenomena  and  what  may  be  expected  therefrom.  The  mining 
engineer,  on  the  other  hand,  is  first  and  last  concerned  with  the 
question  of  profitable -operation. 

In  mine  examination  for  valuation,  only  such  geological  features 
need  be  investigated  as  have  a  direct  influence  on  the  ore  deposit, 
and  under  ordinary  circumstances  the  general  geology  of  the 
district  has  no  direct  influence  on  the  value  of  the  mine.  Occasion 
may  arise  for  the  engineer  to  go  some  distance  afield,  in  order  to 
secure  evidence  bearing  on  the  genesis  of  the  deposit,  or  confirm 
observations  made  underground,  but  in  general  it  may  be  stated 
that  only  the  immediate  geological  aspects  of  the  deposit  need  be 
considered.  If  we  had  an  orebody  completely  outlined  and  de- 
limited by  development  openings,  its  geology  would  be  of  no  con- 
sequence to  the  mine  valuer.  It  is  only  when  questions  of  doubt 
arise  as  to  the  continuity  of  ore  that  the  geological  viewpoint  must 
be  seriously  considered. 

Aside  from  the  association  of  precious  metals  with  certain 
minerals  in  the  orebody,  the  value  of  the  ore  may  be  affected  by 
the  nature  of  the  surrounding  rock,  by  faults  and  dislocations,  by 
fracturing  and  fissuring,  and  by  other  well  recognized  phenomena. 
The  influences  of  secondary  enrichment  often  have  a  marked  bear- 
ing. The  character  of  the  formation  in  which  the  deposit  occurs 
is  of  great  importance  and  is  perhaps  the  first  geological  factor  to 
receive  attention.  What  kind  of  rock  surrounds  the  orebody?  At 
the  surface,  rocks  are  more  or  less  weathered  and  if  the  weathering 


GEOLOGICAL  FACTORS  51 

is  at  all  pronounced,  it  becomes  difficult  to  classify  a  rock  from 
optical  inspection.  I  have  seen  expert  geologists  at  a  loss  to 
determine  a  rock  under  such  circumstances. 

The  mining  engineer  is  generally  less  expert  in  this  work,  yet 
in  an  attempt  to  show  profound  knowledge,  or  in  order  to  attain 
an  accuracy  that  is  entirely  unnecessary,  he  often  over-reaches 
himself  and  shows  ignorance.  To  prove  this,  all  one  has  to  do  is 
to  take  two  or  three  reports  on  one  property  by  different  engineers, 
and  it  will  usually  be  found  that  if  there  is  any  chance  to  call  the 
rock  formation  anything  else  than  it  really  is,  it  will  be  done.  At 
any  rate,  each  engineer  is  likely  to  call  the  rock  by  a  different 
name,  although  the  financial  side  of  the  business  may  find  all  of  them 
in  accord.  The  character  of  the  rock  formation  and  the  kind  of 
rock  is  not  by  any  means  a  guide  to  the  persistence  or  richness  of 
an  ore  deposit;  and  after  all  the  important  question  is,  can  it  be 
profitably  mined? 

I  do  not  mean  to  deprecate  the  usefulness  of  careful  geological 
work  to  the  mine  valuer,  but  merely  wish  to  point  out  that  a 
detailed  geological  investigation  and  accurate  classification  of  the 
rocks  may  be  unnecessary  and  serve  no  useful  purpose  in  the  work 
of  determining  the  worth  of  the  mine.  If  it  can  be  done,  so  much 
the  better.  On  the  other  hand,  when  an  engineer  is  in  doubt,  his 
purpose  will  ordinarily  be  sufficiently  served  by  the  use  of  the 
following  eight  general  groups : 

Class.  Kind. 

T  j    /.     Granitic  Rocks. 

I  2.     Porphyritic  Rocks 

Volcanic  3.     Lavas — Light  and  dark 

i  ; 

6.    Trap  Rocks 

Metamornhir  \   7~     Schists    (including  Gneiss) 

\  8.     Shales  and   Slates 

It  is  only  in  rare  cases  that  the  geological  age  is  of  the  slightest 
interest  in  mine  examination  work.  We  know  that  in  certain 
districts,  gold  deposits  ordinarily  occur  in  rocks  of  a  particular  age, 
but  this  is  no  proof  that  similar  deposits  in  other  districts  will 
yield  a  profit,  nor  even  that  all  deposits  in  the  same  field  are  of 
value.  As  the  Cornishman  says :  "Where  it  is,  there  it  is." 

One  of  the  most  common  statements  made  by  people  who  have 
mines  for  sale  is  that  the  deposit  is  geologically  the  same  in  all 
respects  as  some  famous  producer.  In  the  United  States  for  years 
attempts  have  been  made  to  interest  capital  in  some  occurrences 
of  native  copper  in  trap  rocks  in  the  Shenandoah  Valley  of  Virginia. 
It  is  claimed  for  them  that  they  are  geologically  the  counterpart 


52  MINE    SAMPLING    AND   VALUING 

of  the  famous  Calumet  and  Hecla  lode.  And  they  are,  too,  so  far 
as  the  rock  goes,  but  unfortunately  the  copper  does  not  occur  in 
commercial  quantities. 

During  the  South  African  boom  it  was  claimed  that  hundreds 
of  miles  away  other  deposits  similar  to  the  Rand  banket  had  been 
discovered.  The  West  African  banket  forms  a  more  recent  spec- 
tacular illustration  of  the  use  that  is  made  of  similarity  of  forma- 
tion. The  difference  in  the  value  of  the  two  deposits  is  well  known. 

Another  assertion  commonly  made  is  that  the  property  being 
offered  is  on  the  same  strike  as  some  other  lode  miles  away,  or  that 
the  ore  opened  up  is  a  continuation  of  the  famous  mine  on  the 
other  side  of  the  mountain.  Such  a  statement  may  or  may  not  be 
true.  If  true,  it  does  not  necessarily  increase  the  value  of  the  ground 
under  consideration.  All  orebodies  have  a  definite  length  and 
speaking  generally,  beyond  the  profitable  zone,  there  is  little  like- 
lihood of  finding  another  pay  shoot,  so  that  the  prospect  must  not 
be  overvalued  on  account  of  its  proximity  to  a  highly  developed  and 
prosperous  mine  in  the  neighborhood.  Nevertheless,  one  of  the 
greatest  aids  to  the  examining  engineer  is  that  afforded  by  studying 
other  mines  in  the  same  district. 

It  may  be  taken  as  an  axiom  that  in  any  given  mining  district, 
the  same  kind  of  deposit  occurring  under  the  same  geological  con- 
ditions, has  been  formed  contemporaneously  with  its  neighbors  and 
geologically  may  be  expected  to  yield  similar  results  with  similar 
development,  although  this  does  not  necessarily  mean  the  same 
financial  results.  If  we  refer  to  the  history  of  Butte,  Montana, 
we  find  that  the  gossan  outcrops  were  formerly  mined  for  their 
silver  content  and  the  presence  of  copper  in  the  deposit  was  un- 
suspected for  some  years.  On  further  development  these  outcrops 
proved  to  be  leached  copper  gossan  and  in  depth  the  copper  which 
had  migrated  from  above  was  found  re-deposited  in  a  highly  con- 
centrated form,  the  ores  consisting  of  chalcocite,  covellite,  bornite, 
and  other  minerals  rich  in  copper.  The  recurrence  of  the  same 
phenomena  in  similar  deposits  in  that  district  is  a  natural  and 
justifiable  deduction,  but  is  not  sufficient  reason  for  assuming  that 
a  silver-bearing  gossan  200  miles  away,  would  also  turn  into  high- 
grade  copper  ore  in  depth.  The  gossan  may  easily  have  resulted 
from  the  weathering  of  a  body  of  iron  pyrite,  with  which  no  copper 
was  associated. 

The  importance  of  the  geological  evidence  must  be  decided  on 
its  merits,  and  the  keen  observer  may  be  able  to  see  further  into  the 
ground  than  another  less  observant  individual.  It  is  only  partly 
true  that  no  man  can  see  further  than  the  point  of  his  pick.  Some 
men  see  and  properly  estimate  indications  unseen  by  others.  I 
know  of  a  mine  worked  to  apparent  exhaustion  by  one  manager, 
who  recommended  his  directors  to  abandon  the  property,  whereas 
his  successor,  after  studying  the  formation,  drove  a  cross-cut  less 
than  20  ft.  and  the  property  has  been  paying  dividends  for  the  eight 
or  ten  years  that  have  since  elapsed.  In  mine  valuation  it  is  not  often 


GEOLOGICAL  FACTORS  53 

that  we  dare  capitalize  indications  of  that  kind,  but  in  a  case  where 
the  purchase  is  warranted  by  the  ore  reserves,  a  line  of  develop- 
ment may  be  recommended  to  the  eventual  benefit  of  the  engineer's 
clients. 

Many  geological  problems  arise  to  perplex  the  mine  valuer.  In 
large  mines,  where  the  character  of  the  orebody  may  be  carefully 
studied  in  many  hundreds  or  thousands  of  feet  of  development  open- 
ings, the  problem  is  simplified.  The  evidence  is  not  always  com- 
plete, yet  the  constructive  type  of  engineer  likes  to  form  a  definite 
opinion  as  to  the  future  possibilities  of  the  mine.  In  other  words, 
he  wants  to  feel  that  the  mining  risk  assumed  by  his  clients  will  be 
justified  by  the  subsequent  developments.  No  man  cares  to  under- 
take a  business  that  will  merely  return  his  original  investment.  The 
greatest  skill  is  called  forth  in  remote  districts  where  the  engineer's 
sole  guide  is  his  experience  and  there  are  no  neighboring  mines  to 
furnish  corroboratory  evidence.  Under  such  circumstances  I  have 
known  more  than  one  engineer  to  condemn  a  mine  sooner  than  risk 
his  reputation.  Such  practice  is  more  than  reprehensible. 

With  a  mine  having  perhaps  a  large  amount  of  development 
work  above  water  level  and  the  purchase  price  warranted  by  the 
ore  reserves,  a  close  study  of  the  geology  is,  nevertheless,  essential. 
The  engineer  is  called  upon  to  determine  not  only  the  likelihood  of  the 
persistence  of  the  ore  below  the  water  level,  but  also  whether  its 
mineralogical  character  will  so  change  as  to  render  profitable  treatment 
impossible.  In  the  oxidized  zone  of  gold  deposits  the  presence  of 
arsenic  and  antimony  is  often  unsuspected  as  is  zinc  in  carbonate  of 
lead  ores.  Aside  from  the  question  of  secondary  enrichment  discussed 
later,  the  liability  of  certain  metals  to  concentrate  at  particular  horizons 
must  be  ever  kept  in  view. 

On  study  it  may  be  shown  that  the  valuable  ore  is  genetically  con- 
nected with  the  intrusion  of  dikes,  in  fact,  the  dikes  may  have  been 
the  cause  of  the  fissuring.  Cross-dikes  may  cut  off  valuable  ore  alto- 
gether or  may  mark  the  channel  along  which  the  enriching  solutions 
or  gases  penetrated,  so  that  profitable  ore  is  only  found  at  the  inter- 
section of  the  vein  and  dike ;  then,  too,  they  may  cause  faulting  of  the 
lode  and  sometimes  an  impoverishment  of  the  vein.  The  selective 
action  of  ore-bearing  solutions  is  well  known,  and  this  is  most  marked 
in  limestone  formations,  where  perhaps  a  seam  may  have  acted  as  a 
conduit  for  the  solutions  which  have  spread  out  in  one  of  the  more 
soluble  beds  to  form  an  orebody  lying  conformably  to  the  stratifica- 
tion. This  same  phenomenon  may  be  repeated  at  other  points. 

It  is  also  well  known  that  the  same  lode  in  passing  from  one  kind 
of  rock  to  another  may  vary  in  the  character  of  its  mineral  content. 
The  change  from  copper  to  tin  in  Cornwall  is  well  known.  Another 
illustration  is  the  lode  formation  at  Silver  Islet  in  Lake  Superior,  which 
contained  a  wonderful  deposit  of  native  silver  at  that  point,  but  on  the 
mainland  the  same  vein  in  different  country  rock  contains  no  silver. 
The  Camp  Bird  mine,  Colorado,  is  another  case  in  point,  where  devel- 
opment below  a  certain  horizon  proves  fruitless.  The  valuing  engineer 
must  study  the  formation  to  learn  of  the  likelihood  of  the  lode  passing 


54  MINE    SAMPLING   AND   VALUING 

into  rock  of  a  different  character  and  the  influence  that  such  change 
may  have  on  its  value.  In  the  monograph  on  Goldfield,  Nevada,  pre- 
pared by  eminent  geologists,  and  issued  by  the  U.  S.  Geological  Survey, 
the  statement  is  made  that  in  passing  from  one  rock  to  another  the 
value  was  likely  to  decrease.  Subsequent  work  has  proved  this  theory 
to  be  incorrect,  and  it  is  quoted  as  an  illustration  that  unless  the  evidence 
is  positive,  it  is  unwise  to  draw  conclusions  of  this  kind.  The  main 
proof  after  all  is  the  development  work  and  it  would  seem  that  usually 
a  statement  to  the  effect  that  valuable  ore  will  not  continue  beyond  a 
certain  horizon  is  apt  to  be  falsified  in  cases  where  such  a  statement  is 
based  on  theory  instead  of  the  evidence  of  actual  underground  workings. 

The  geology  of  contact-metamorphic  deposits  is  particularly 
puzzling  and  the  literature  on  the  subject  shows  that  duplicate  geological 
conditions  do  not  imply  the  same  commercial  value.  Much  has  been 
written  on  the  geology  of  ore  deposits  and  justice  cannot  be  done  in 
the  few  pages  devoted  to  it  here.  In  solving  the  problems  that  present 
themselves,  experience  and  judgment  are  necessary. 

There  are  a  number  of  useful  works  on  Economic  Geology  that  may 
be  perused  with  benefit  by  the  mine  valuer.  A  knowledge  of  ore  de- 
posits is  an  essential  qualification  to  him ;  and  although  the  ability  to 
accurately  classify  rocks  in  the  field  is  not  of  the  first  importance,  it 
is,  of  course,  of  some  satisfaction  to  the  engineer  to  be  able  to  do  so. 
The  table  on  p.  55  is  republished  from  the  Mining  and  Scientific  Press, 
vol.  99,  p.  599. 

There  is  a  maxim  in  mining  that  practically  all  ore  deposits,  except 
those  of  iron,  are  subject  to  the  influences  of  secondary  enrichment, 
and  that  in  the  zone  of  secondary  enrichment  ore  of  higher  value  will 
be  obtained  than  below  it. 

The  effects  of  secondary  enrichment  on  copper  deposits  are  of  course 
marked,  and  have  been  so  active  as  generally  to  have  removed  prac- 
tically all  the  copper  from  the  portion  of  the  orebody  lying  near  the  sur- 
face, leaving  a  well  marked  gossan.  Below  this  mass  of  oxidized 
material  is  a  zone  of  greater  or  less  extent,  containing  a  concentration 
of  the  valuable  metals,  often  20  or  30  times  as  rich  as  the  original 
unaltered  material,  which  when  penetrated  may  be  found  to  be  of  no 
commercial  value.  The  low-grade  disseminated  copper  deposits  are 
startling  illustrations  of  this  fact. 

Again  the  history  of  gold  mining  demonstrates  that  secondary  en- 
richment is  an  important  factor  in  raising  the  assay  value  of  the  ore 
lying  near  the  surface  to  an  important  extent,  for  we  find  that  all  gold 
mines  eventually  pass  out  of  the  higher  grade  of  ore,  which  has  been 
the  cause  of  early  success,  into  ore  of  considerably  lower  grade.  In 
the  case  of  gold  mines,  however,  the  change,  though  well  marked,  is 
riot  so  great  as  is  ordinarily  the  case  with  copper  mines,  and  provided 
the  mine  has  been  properly  equipped  with  efficient  plant,  profitable 
operations  may  usually  be  continued  for  a  considerable  further  period. 

The  object  of  pointing  out  these  facts  is  to  draw  the  attention  of 
the  engineer  to  the  necessity  of  keeping  them  in  mind  during  sampling 
operations,  and  he  should  endeavor  to  secure  information  bearing  on 
this  point.  If  a  gradual  enrichment  is  found  from  the  surface  to  the 


GEOLOGICAL  FACTORS 


IGNEOUS  BOCKS. 

PLUTONIC.                                                INTERMEDIATE. 

ERUPTIVE. 

GRANITE-RHYOLITE  SERIES. 

Granites. 
Holocrystalline.     Dominant    miner- 
als:    quartz     and     alkali     feldspar 
(orthoclase  or  microcllne).     Subor- 
dinate minerals:  muscovite,  biotite, 
hornblende,  etc. 
Aplite.  a  granite  with  quartz  and  feld- 
spar only. 

Quartz  Porphyries. 
Intermediate  between  the  granites 
and    rhyolites. 

Rhyolites. 
Eruptive  equivalents  of   the  gran- 
ites with  same  chemical  composi- 
tion.    Quartz    and    feldspar,    pre- 
dominating    minerals.     Commonly 
contain    more    or    less    undifferen- 
tlated  glass. 
Obsidian,   is  the  wholly  vitreous   va- 
riety of  rhxollte. 

The  difference  between  granites  and  rhyolltes  are  structural  and  genetic;   chemically  and  magmatically 
they  are  the  same. 

SYENITE-TRACHYTE  SERIES. 

The  Syenite-Trachyte  series  differs  from  the  Granite-Rhyolite  series  In  being  free,  or  nearly  so,  from  quartz. 
All  of  these  rocks  contain  principally  alkali  feldspars,  with  subordinate   femic   minerals,   and   often   alferric   species, 
such  as  hornblende,  mica,  etc. 

Syenites. 
Resemble    the    granites    in    their 
deep-seated  plutonlc  origin  and  in 
being  holocrystalllne. 

Syenite  Porphyries. 
Intermediate    forms    between    the 
syenites   and    trachytes,   analogous 
to  the  quartz  porphyries. 

Trachytes. 
Like  the  rhyolltea,  they  are  .erup- 
tive rocks. 

NEPHELITE  SERIES. 

These  are  transition  rocks  from  the  syenites  and  trachytes   proper,   to   the   phonolites   and   nephelfne   syenites. 
They  occur  in  plutonic,  intermediate,  and'eruptive  groups  like  those  abovev    Quartz  is  absent,  and  lenads,  or  feld- 
spathoids  (feldspars  deficient  in  silica)  replace  the  feldspars  to  a  greater  or  less  extent. 
Phonoltte  Is  commonly  made  up  of  orthoclase,  nephelite,  and  pyroxene. 

MONZONITE  GROUP. 

The  Granite-Rhyolite  series  of  rocks,  and  the  Syenite-Trachyte  series  also,  are  defined  by  the  predominance  in 
them  of  alkali  feldspars,  and  commonly  of  orthoclase. 
The  Andesite-Dlorite  se.'les   (see  below)    is  characterized  by  plagioclase  feldspars. 
Between  these  series  are  all  sorts  of  gradations  known  as  monzonites.    All  these  rocks  carry  orthoclase  or  anortho- 
clase  and  plagioclase  in  approximately  equal  amounts,  with  or  without  quartz,  and  with  smaller  amounts  of  the  ferro- 
magnesian  silicates. 

Quartz  Monzonite  corresponds  with  granite.                            Latite  is   an   effusive,  equivalent,   intermediate  between 
'Monzonite  corresponds  with  syenite.'                                                the  trachytes  and  andesites. 

ANDESlTE-DIORITE  SERIES. 

From  the  monzonite  group  to  the  quartz-diorites  the  gradation  is  very  slight.     These   rocks,   which  mark  the 
persilicic  end  of  the  Andeslte-Diorite  series,  are  characterized  by  quartz,  w,ith  plagioclase  as  the  prevailing  feldspar, 
and  with  subordinate  amounts  of  ferric  minerals.    They  correspond  to  granite  and  rhyollte  in  the  Orthoclase  series. 

Quartz  Diorites. 
Plutonic  or  deep  seated  like  granite. 
Diorites. 
Plutonic  equivalent  of  andeslte.    A 
granitoid  rock  consisting  chiefly  of 
plagioclase   with   either   biotite   or 
hornblende,  or  both.    Many  diorites 
carry    pyroxenes    and    shade    into 
gabbros. 

Diorite  Porphyries. 
Analogous  to  the  quarts  porphyries. 

Dacites. 
Eruptive  like  rhyollte.    Dacite  is  a 
quartz  andesite. 
Andesites. 
Poor  or  lacking  in  quartz.     They 
form  a  group  of  rocks  parallel  to 
the  trachytes,  and  contain  plagio- 
clase   as    a    principal    constituent, 
with     subordinate     biotite',     horn- 
blende, and  pyroxene. 

GABBROS. 

DIABASE. 

BASALTS. 

Granitoid  equivalent  of  the  basalts. 
Consist  mainly  of  plagioclase  and 
pyroxenes,  with  various  admixtures 
of  other  minerals.     A  large  family 
of  rocks,  varying  from  almost  en- 
tirely plagioclase  to  near  pure  femic 
rocks,    such   as    pyroxenites,    horn- 
blendites,  and  peridotltes. 

Intermediate  in  texture  between  the 
granitoid  gabbros  and  the  basalts. 
Consists  chiefly  of  plagioclase,  aug- 
ite,  magnetite,  and  sometimes  oliv- 
ine. 

Contain  more  femic  minerals  than 
andesites.     Plagioclase,     pyroxene, 
magnetite,    and    often    ollvlne    are 
principal  constituents.    Hornblende 
rarely  occurs. 

0 

56  MINE    SAMPLING   AND   VALUING 

bottom  level  of  the  mine,  when  assuming  a  value  for  the  probable  ex- 
tension of  ore  in  depth,  due  weight  must  be  given  to  the  likelihood  or 
otherwise  of  exposed  ore  continuing,  and  the  influence  which  secondary 
enrichment  has  had.  In  arriving  at  a  decision,  the  geological  factors 
must  be  carefully  taken  into  consideration.  There  are  numerous  in- 
stances where  long  shoots  of  ore  of  good  grade  have  been  opened  and 
have  extended  to  a  depth  of  only  200  or  300  ft.  below  the  surface,  to  be 
replaced  by  unprofitable  material.  The  Vivien  and  Bellevue  mines  in 
Western  Australia  are  illustrations  of  this  fact.  Many  others,  of  course, 
could  be  cited. 


CHAPTER  VII. 

CHURN  DRILLING  AS  APPLIED  IN  SAMPLING. 

There  are  deposits  of  copper,  iron,  lead,  and  zinc  that  occur  in 
tabular  or  massive  form,  whose  greatest  dimensions  are  their  superfi- 
cial area,  and  which  crop  out  at  the  surface,  or  are  so  near  to  it  that 
they  can  be  economically  penetrated  by  means  of  mechanically  operated 
drills,  such  as  diamond  drills  and  churn  drills. 

For  many  years  the  huge  deposits  of  iron  ore  in  the  Lake  Superior 
region  have  been  sampled  by  means  of  diamond  drills.  More  recently 
the  churn  drill,  long  used  for  drilling  oil  wells,  has  been  applied  for 
sampling  the  disseminated  copper  ores,  from  which  such  an  important 
proportion  of  the  American  production  is  now  being  obtained.  In  the 
zinc  and  lead  districts  of  Missouri  and  Kansas,  similar  methods  have 
been  in  successful  use. 

The  churn  drill  has  largely  supplanted  the  diamond  drill  where  a 
vertical  hole  can  be  put  down,  because  it  is  cheaper  to  operate  and 
gives  a  more  satisfactory  sample.  The  diamond  drill  hole  is  1  to  2 
inches  in  diameter,  whereas  the  churn  drill  puts  down  a  hole  6  to  12 
inches  in  diameter,  so  that  a  much  larger  sample  is  obtained.  The  dia- 
mond drill  is  limited  to  rock  sufficiently  resistant  to  yield  a  solid  core, 
but  on  the  other  hand  it  has  the  advantage  of  being  able  to  drill  a  hole 
at  any  angle. 

It  is  not  proposed  to  discuss  diamond  drill  methods,  as  these 
are  well  understood,  and  the  principal  makers  will  always  contract 
work;  besides,  the  churn  drill  has  come  into  general  use  where 
vertical  holes  can  be  put  dowrn. 

No  discussion  of  mine  sampling  at  the  present  day  would  be 
complete  without  some  discussion  of  the  methods  employed  in 
churn-drill  sampling  of  that  class  of  deposits,  which  were  formerly 
known  as  low-grade  porphyry  copper  deposits,  but  are  now  usually 
spoken  of  as  disseminated  copper  deposits.  These  deposits  are 
commercially  important,  and  though  low  grade,  are  worked  on  a 
scale  hitherto  unattempted,  so  that  they  now  are  an  important 
factor  in  the  copper  production  in  the  United  States.  The  ores 
occur  as  chalcocite  derived  from  the  leaching  of  copper  in  primary 
chalcopyrite  impregnations  in  porphyritic  and  schistose  rocks  and 
its  redeposition  at  a  lower  horizon  in  the  richer  form  in  which  it 
is  found.  The  enriched  part  usually  has  a  vertical  depth  of  not 
less  than  200  ft.  and  lies  more  or  less  parallel  to  the  surface  of  the 
ground.  The  leached  material  between  it  and  the  surface  consti- 
tutes what  is  known  as  the  overburden  and  sometimes  is  suffi- 
ciently shallow  to  warrant  steam-shovel  operations.  Beneath  the 
zone  of  enrichment  is  unprofitable  material,  containing  the  origi- 
nal unaltered  sulphides. 


58  MINE    SAMPLING   AND   VALUING 

Many  deposits  of  this  character  have  been  prospected  in  the 
United  States  by  the  use  of  churn  drills,  and  enormous  tonnages 
proved  by  systematic  drilling  operations.  Two  types  of  drill  have 
been  thoroughly  tested;  the  Star  and  Keystone,  each  possessing 
particular  advantages  in  certain  rocks.  The  prospective  mineral- 
ized area  is  co-ordinated  into  200-ft.  blocks,  that  is,  divided  up 
checker-board  fashion  and  holes  put  down  at  the  intersection  of 
coordinates.  They  are  logged  and  mapped  according  to  the  dis- 
tance from  the  datum  lines,  as  for  example :  North  400,  East  800,  or 
N4,  E8.  Barring  accident,  the  hole  is  continued  until  the  zone  of 
primary  sulphides  is  proved  beyond  doubt.  The  holes  are  sampled, 
as  will  be  later  described,  in  running  five-foot  sections,  while  pan- 
ning tests  are  made  at  every  sludging  of  the  hole  to  determine  not 
only  the  character  of  the  rock  being  drilled  but  also  the  nature  of 
contained  mineral.  While  running  through  the  barren  oxidized 
overburden,  samples  for  assay  are  only  taken  every  20  or  30  ft. 
By  means  of  these  machines,  which  are  modified  oil-well  drills,  a 
cylindrical  section  from  12  to  A-l/2  in.  diameter,  according  to  depth, 
is  taken,  from  the  surface  to  the  bottom  of  the  hole. 

The  general  method  of  sampling  is  much  the  same  in  different 
parts  of  the  country.  For  most  of  the  following  description  I  am 
indebted  to  Lloyd  T.  Buell  of  the  Miami  Copper  Co.'s  staff  and  to 
Frank  H.  Probert,  consulting  engineer,  Los  Angeles,  California. 

The  mechanical  operation  of  obtaining"  a  sample  consists  of  four 
steps:  (1)  cutting  the  sample;  (2)  removing  the  cuttings  from 
the  hole;  (3)  reducing  the  sample  to  convenient  size;  (4)  drying. 

The  actual  drilling  will  not  be  described  in  detail,  but  the  errors 
which  may  affect  the  sample  will  be  considered  later.  During  the 
process  of  drilling,  water  is  poured  into  the  hole  as  needed  to  form 
with  the  cuttings  a  thin  mud  or  sludge,  and  as  the  tools  are  being 
removed  from  the  hole  the  mud  adhering  to  them  is  washed  back 
into  the  hole.  The  cuttings  from  five  feet  of  hole  constitute  one 
sample,  though  it  is  sometimes  necessary  to  remove  the  tools  and 
clean  the  hole  more  than  once  in  advancing1  that  distance.  The 
depth  from  which  a  sample  is  cut  is  read  from  measurements 
marked  on  the  drilling  cable  as  the  hole  is  drilled. 

The  cuttings  are  recovered  by  a  bailer  which  discharges  at  the 
surface  into  a  launder,  several  trips  being  necessary  each  time  the 
hole  is  cleaned.  This  bailer  which  lifts  the  pulp  from  the  bottom 
of  the  hole  is  operated  by  a  dart  valve,  which  prevents  the  sludge 
from  splashing  over  the  sides  of  the  hole  as  it  is  withdrawn.  It 
is  only  released  by  the  dart  striking  the  bottom  of  the  launder 
which  conveys  the  pulp  to  a  split  sampler.  At  Miami  this  sampler, 
which  in  effect  is  a  multiple  Jones  sampler,  is  made  by  the  com- 
pany's tinsmith.  Part  of  the  discarded  pulp  from  the  sampler  is 
caught  in  a  bowl  and  carefully  panned.  It  is  examined  by  means 
of  a  hand  lens  to  record  the  nature  of  the  rock  and  minerals  passed 
through.  The  results  are  entered  in  the  log.  The  pulp  as  it  passes 
through  the  sampler  is  divided  three  times,  one-half  being  rejected 


CHURN    DRILLING  59 

each  time,  so  that  one-eighth  of  the  material  recovered  from  the 
hole  is  reserved  as  a  final  sample  and  collected  in  a  galvanized  iron 
tub,  usually  about  30  inches  in  diameter. 

The  tub  is  then  set  over  an  open  fire  and  the  water  boiled  away 
and  the  sample  dried  and  sacked.  It  is  then  sent  to  the  laboratory, 
where,  after  it  has  been  pulverized  and  successively  reduced  until 
the  final  pulp  is  obtained,  it  is  ready  for  analysis.  As  a  general 
average,  the  total  amount  of  dry  pulp  of  each  representative  five- 
foot  sample  is  about  40  Ib.  If  the  ground  is  caving,  it  may  happen 
that  the  dried  sample  obtained  is  too  large  to  go  in  one  sack,  in 
which  case  at  Miami  it  is  coned  and  quartered,  or  otherwise  divided 
at  the  drill  to  reduce  it  to  convenient  size. 

The  errors  peculiar  to  churn-drill  sampling  may  be  classified 
as  follows:  1.  Deviation  of  the  hole  from  the  vertical.  2.  Separa- 
tion from  the  rock  at  the  point  of  drilling  of  valuable  minerals,  re- 
sulting in  a  sample  that  is  too  rich.  3.  Concentration  by  gravity 
of  heavy  minerals  in  the  bottom  of  the  hole  so  that  they  are  not 
recovered  when  the  hole  is  cleaned,  but  remain  in  the  hole  and 
follow  it  down  to  be  recovered  at  a  lower  level.  4.  Caving  of 
rock  from  the  sides  of  the  hole. 

The  character  of  the  ground  in  most  of  the  disseminated  copper 
properties  is  such  that  careful  drilling  will  usually  insure  a  vertical 
hole,  deflection  being  caused  by  the  drill  encountering  harder  rock 
at  an  acute  angle. 

Concentration  of  minerals  in  the  bottom  of  the  hole  by  gravity 
tends  to  equalize  the  grade  of  the  samples  and  to  indicate  the  oc- 
currence of  the  heavy  minerals  somewhat  below  their  greatest 
depth.  Theoretically,  the  extent  to  which  such  concentration  will 
take  place  will  depend  upon  the  size,  hardness  and  specific  gravity 
of  the  mineral  particles,  and  will  be  minimized  by  careful  cleaning 
of  the  hole  each  time  the  cuttings  are  removed,  and  a  knowledge 
of  the  manner  of  occurrence  of  the  ore  will  prevent  a  misinterpre- 
tation of  the  results. 

Caving  is  the  most  obvious  cause  of  error  in  churn-drill 
sampling  and  may  entirely  vitiate  the  results.  The  remedy,  how- 
ever, is  simple  and  effective  and  consists  in  lowering  casing'  to  the 
bottom  of  the  hole  and  proceeding  with  a  smaller  bit,  repeating  the 
operation  three  or  four  times  if  necessary,  each  time  using  smaller 
casing  and  a  smaller  bit.  Extensive  caving  will  be  indicated  to 
the  driller  by  the  feel  of  the  tools  while  drilling.  Comparatively 
slight  falls  of  rock  may  be  detected  in  this  way,  or  the  caving  may 
be  so  great  as  to  cause  difficulty  in  extracting  the  tools.  The  re- 
covery of  more  sludge  than  would  be  derived  from  the  advance 
made  or  the  presence  in  the  sludg'e  of  material  foreign  to  the  ground 
being  cut  are  evidences  of  caving.  Casing  is  frequently  necessary 
simply  to  keep  the  hole  open  so  that  drilling  can  continue,  regard- 
less of  the  validity  of  the  samples  secured.  In  fact  this  is  practi- 
cally always  necessary  in  deep  holes.  Frequently  when  casing  has 


60  MINE    SAMPLING   AND   VALUING 

not  been  used  it  is  lowered  as  soon  as  the  ore  is  reached  as  a 
preventive  measure,  whether  any  caving  has  taken  place  or  not. 

Permanent  records  for  each  hole,  known  as  the  log,  show  the 
kinds  of  rock  passed  through  and  the  limits  of  each,  the  important 
minerals  present  and  the  grade  and  character  of  the  ore,  the  latter 
by  five- foot  sections;  also  all  details  of  the  drilling  operations,  in- 
cluding1 mention  of  caving  if  it  occurs,  and  the  amount  and  size  of 
casing  used.  The  examination  for  minerals  is  made  with  the  aid 
of  a  vanning  plaque  at  the  drill  as  each  sample  is  taken  from  the 
hole.  In  this  connection  it  should  be  noted  that  excessive  heat  in 
drying  the  sample  after  the  water  has  been  boiled  away  will  result 
in  the  oxidation  of  some  of  the  sulphide  minerals,  and  the  sample 
must  be  dried  slowly  if  an  accurate  determination  of  these  minerals 
from  the  dried  sample  is  important. 

Taking  the  usual  practice  in  the  United  States,  the  hole  at  the 
surface  is  usually  from  I0y2  to  12  inches  in  diameter  and  the 
average  diameter  at  the  bottom  would  be  8  to  8j4  inches,  so  it  will 
be  seen  that  a  large  sample  is  obtained. 

According  to  Mr.  Buell  the  churn-drill  results  at  Miami  at  the 
time  he  wrote  had  not  yet  been  proved  by  the  underground  devel- 
opment work,  however  in  other  parts  of  the  country  the  under- 
ground developments  have  checked  the  results  obtained  by  churn 
drilling  to  a  remarkable  extent. 

In  the  autumn  of  1909  I  had  the  opportunity  of  seeing  the  actual 
results  of  the  sampling  of  the  Ray  Consolidated  mine.  A  great 
many  of  the  drill  holes  put  down  were  tested  by  means  of  rises 
put  up  on  the  line  of  the  hole,  half  of  the  hole  being  carried  in  one 
corner  of  the  rise.  Individual  results  vary  considerably  in  some 
cases,  but  taken  over  a  series  of  holes  the  average  values  obtained 
from  the  churn-drill  samples  and  samples  taken  in  the  rises  in  the 
ordinary  manner  check  to  thousandths  of  one  per  cent.  The  ex- 
perience then  may  be  stated  to  be  that  in  sampling  deposits  of  this 
kind  individual  results  may  not  be  depended  upon  any  more  than 
in  the  ordinary  ore  .deposits.  However,  owing  to  the  compara- 
tively even  distribution  of  the  valuable  mineral  in  the  gangue, 
samples  are  not  required  to  be  taken  with  the  same  frequency  as 
with  the  usual  type  of  deposits. 

The  description  of  the  log  given  above  is  that  used  at  Miami. 
Mr.  Probert  devised  a  graphic  method  of  presenting  all  the  essen- 
tial details  of  churn-drilling  work  at  the  Ray  Central  mine  in 
Arizona.  This  is  shown  in  the  accompanying  illustration  (Fig.  13). 
This  graphic  log  presents  all  the  required  information  at  a  glance 
in  a  much  more  lucid  manner  than  is  possible  from  a  mere  inspec- 
tion of  figures,  and  Mr.  Probert  deserves  great  credit  for  de- 
vising it. 

So  much  confidence  is  placed  in  the  results  obtained  by  churn 
drilling  that  it  is  customary  to  reckon  as  proved  all  are  included 
within  holes  200  ft.  or  less  apart.  If  holes  are  set  at  greater  dis- 


CHURN    DRILLING 


61 


tances,  if  systematically  drilled  the  included  ore  is  counted  as  partly 
proved  or  probable  ore.  Already  millions  of  tons  have  been 
mined  from  areas  tested  in  this  manner,  with  results  so  satisfac- 
tory as  to  warrant  the  continuance  of  the  practice.  That  ground 
sampled  in  this  manner  should  be  accepted  as  proved  ore  or 


GEOUOGV 


•ASSAY  CURVE 


r— ASSAYS 


UNCRALS— , 


Fig.    13.     CHURN-DRILL  LOG. 


62 


MINE    SAMPLING    AND   VALUING 


blocked-out  ore  is  a  startling  demonstration  of  the  fact  that 
experience  and  judgment  are  needed  by  the  mine  valuer.  Com- 
bined with  the  interpretation  of  the  drill-hole  assays,  a  knowledge 
and  comprehension  of  the  geological  conditions  is  essential.  Only 
the  comparatively  uniform  distribution  of  the  valuable  mineral 
over  a  large  area  renders  it  possible  to  accept  the  results  as  suffi- 
ciently conclusive  for  valuation  purposes.  At  the  Giroux  mine, 
Ely,  Nevada,  in  correlating  the  assay  results,  averages  were  cal- 
culated not  only  along  the  coordinates  but  also  along  the  diago- 
nals,-to  arrive  at  a  closer  check  in  cases  where  the  results  of  ad- 
joining holes  showed  considerable  variation  in  assay  values. 

In  Arizona  the  average  rate  of  advance  in  drilling  per  12-hour 
shift  is  30  to  40  ft.  for  the  first  500  or  600  ft.  This  is  somewhat 
better  than  the  results  obtained  at  Ely,  Nevada.  The  costs  in 
both  States  seems  to  be  the  same,  namely,  $2.25  to  $2.50  per  foot,  to 


Fig.    14.     PLATE'S    DEVICE. 


whicli  must  be  added  25c  per  foot  for  road  building  in  mountain- 
ous country, 

Another  method  of  collecting'  a  sample  from  a  churn-drill 
hole  is  described  as  follows  by  H.  R.  Plate:  The  sludge  is  thrown 
directly  into  a  wooden  box  about  6  ft.  long,  4  ft.  wide  and  1  It. 
deep  and  there  allowed  to  settle.  The  clear  water  is  drawn  off  by 
holes  bored  in  the  end  of  the  box  at  different  levels.  When  most 
of  the  water  has  been  removed  the  solid  material  is  sampled  with 
a  split  shovel.  The  amount  of  the  sample,  which  is  about  one 
washtub  full,  is  then  dried  and  treated  in  the  ordinary  way.  The 
objection  to  this  method  of  sampling  is  that  a  portion  of  the 
chalcocite  which  floats  is  lost  in  drawing  off  the  water  and  the 
split-shovel  method  is  not  so  accurate  as  some  of  the  other  meth- 
ods described. 


CHURN    DRILLING  63 

A  device  for  which  Mr.  Plate  is  responsible  and  used  by  him 
at  Ely  is  shown  in  Fig.  14.  It  is  made  of  galvanized  iron  and  is 
about  3  ft.  in  diameter.  The  sludge  enters  at  the  top  and  three- 
quarters  of  the  original  amount  is  rejected,  the  remainder  being 
caught  in  the  tub  and  treated  in  the  manner  already  described. 

H.  R.  Krumb  devised  an  apparatus  that  in  effect  is  a  double 
rifBe  sample,  one  placed  above  the  other.  The  rejected  portion 
flows  out  at  the  ends  of  the  troughs  and  the  quarter  that  goes 
through  is  caught  below. 


CHAPTER  VIII. 

HANDLING  OF  SAMPLES. 

We  have  now  arrived  at  a  stage  in  the  discussion  at  which  it 
may  be  assumed  that  the  sample  has  been  properly  taken,  and  that 
the  broken  mineral  is  to  be  sacked  and  removed  for  assay.  The 
following  suggestions  for  handling  the  sample  are  for  the  guidance 
of  the  younger  men,  although  it  is  hoped  that  even  the  older  men 
may  find  some  points  of  interest. 

Numbering  the  Sample. — The  samples  as  taken  from  consecutive 
intervals  should  be  given  consecutive  numbers.  Some  engineers, 
in  order  to  mislead  the  assayer,  do  not  use  consecutive  numbers. 
This  results  in  a  great  deal  of  extra  work  when  sorting  the  assay 
returns  and  thereby  introduces  another  possibility  of  error. 

The  most  simple  and  flexible  system  of  enumeration  is  to  give 
each  part  of  the  mine  a  different  basic  or  serial  number,  somewhat 
after  the  so-called  decimal  system.  For  instance,  along  No.  1  Level 
North,  we  may  start  off  one  series  as  numbers  1,  2,  3,  etc. ;  then 
101,  102,  103,  etc.,  to  represent  samples,  say,  from  a  particular  stope; 
201,  203,  204,  etc.,  say,  from  a  certain  series  of  winzes  or  rises  and  so 
on  through  the  mine.  This  facilitates  the  work  of  the  examining 
engineer  and  tends  to  prevent  mistakes,  because  in  case  of  error 
the  samples  are  so  easily  traceable. 

The  fractional  samples  in  each  particular  interval  are  marked 
with  the  interval  number  and  going  toward  the  footwall  are  de- 
nominated Fi,  F2,  Fj,  etc.,  and  toward  the  hanging  wall  Hi,  Hz, 
Hj,  etc.  If  there  are  only  two  fractional  samples  from  the  foot  and 
hanging  wall  side  respectively  they  may  be  marked  N.H.  and  N.F.; 
or  N.,  A/77.;  or  Nf  or  N.  H.,  in  each  case  TV  standing  for  the  sample 
number. 

It  will  be  seen  that  this  allows  of  the  introduction  of  additional 
samples  in  any  portion  of  the  mine,  without  making  an  enormous  jump 
in  the  consecutive  numbers.  It  may  be  argued  that  this  system,  al- 
though flexible,  is  cumbersome  on  account  of  the  long  numbers  that 
must  be  used  where  many  parts  of  the  mine  are  given  each  a  separate 
serial  number.  In  my  opinion  it  is  preferable  to  write  a  long  number 
on  a  paper  which  when  seen  immediately  indicates  the  locality  of  the 
sample,  than  it  is  to  have  lower  numbers  and  to  be  under  the  obligation 
of  searching  sampling  records,  in  order  to  determine  its  exact  position. 
For  the  same  reason  the  keeping  of  the  assay  records  is  greatly  sim- 
plified as  the  number  indicates  its  position  in  the  record  book. 

Some  engineers  use  numbered  metallic  disks,  but  these  are  ob- 
jectionable because  they  are  bulky  and  because  there  is  no  flexibility  in 
such  a  system.  The  use  of  metallic  disks  necessitates  giving  a  separate 
number  to  each  fraction,  and  in  this  way  the  nomenclature  is  so  com- 
plicated that  it  is  practically  impossible  for  an  engineer  looking  at  the 


HANDLING  OF  SAMPLES  65 

sample  number  afterwards  to  tell  its  position  underground.  The 
author  has  used  linen  labels  for  a  number  of  years  with  great  success, 
and  has  never  found  a  sample  number  mutilated  to  such  an  extent 
that  there  was  any  difficulty  in  recognizing  it.  Besides  the  flexibility 
in  numbering  that  this  permits,  the  portability  of  these  labels  strongly 
recommends  them.  In  the  absence  of  linen  labels,  strong  bond  paper 
or  drawing  paper  should  be  cut  to  suitable  size. 

Tags  made  of  wood  are  recommended  by  some  engineers.  My  own 
experience  is  that  these  are  very  unsatisfactory.  In  the  first  place, 
they  are  troublesome  to  make,  they  are  bulky,  the  number  is  liable  to 
be  obliterated  in  transport,  or  the  tag  may  even  be  broken  and  crushed 
to  an  extent  to  make  it  difficult  to  read  the  number,  besides  con- 
taminating the  sample  with  the  slivers  of  wood. 

Some  engineers  recommend  putting  the  sample  number  on  the 
bag  for  identification  and  in  order  to  prevent  outsiders  learning  of  the 
identity  of  a  sample  it  has  been  suggested  by  one  man  to  put  the 
number  inside  the  bag,  close  to  the  string.  Numbering  the  bag  is  a 
nuisance  as  it  rarely  arises  that  the  number  of  a  sample  need  be  known 
after  it  is  sacked,  except  where  two  bags  are  used  for  one  sample, 
in  which  case  they  should  be  tied  together. 

If  there  was  any  valid  reason  for  putting  the  sample  number  on 
the  bag,  there  certainly  is  no  excuse  for  concealing  it  by  placing  it  on 
the  inner  side.  This  is  an  admission  of  bad  practice  or  carelessness. 
No  sample  should  ever  be  left  about,  where  it  can  be  seen  by  those 
who  could  take  advantage  of  its  identity.  A  sample  from  the  time 
it  is  taken  to  the  time  it  is  assayed,  should  be  safeguarded  from  inter- 
ference by  outsiders. 

Description  of  Sample.  After  the  sample  has  been  cut,  the 
sampler's  first  duty  should  be  to  make  a  comprehensive  description  of 
the  sample  in  his  notebook.  The  following,  which  is  more  or  less  self- 
explanatory,  is  a  copy  of  a  portion  of  the  author's  sampling  record  on 
one  property : 

Zig  Zag  Workings.*  (See  mine  plan).  1st  long  level — According  to  the 
plan  this  is  about  midway  between  the  Old  'L'  and  No.  2  levels.  At  Rise  670 
floor  is  12  ft.  6  in.  above  the  floor  of  intermediate,  or  about  26  ft.  6  in.  above 
Old  'L'  floor. 

761.  In  west  face  of  26-ft  level— This  is  10  ft.  west  of  west  side  of  Rise 
670.  51  in.  nice  looking  soft  gossany  ore  containing  Mn  and  quartz  and  some 
country  rock.  Neither  wall  reached. 

76-?.  In  drive  of  26-ft.  level,  just  above  west  side  of  Rise  670 — 30  in.  of 
mixed  ore  .from  footwall.  The  bottom  half  of  this  is  almost  completely  replaced 
country  with  some  quartz,  the  remainder  soft  ore  with  quartz,  and  contains 
manganese. 

Hi. — 39-in.  gossany  ore,  mostly  soft,  but  containing  a  large  amount  of  highly 
altered  country  rock  near  hanging  side  of  cut,  whereas  quartz  and  manganese 
predominate  in  other  part  of  sample.  Hanging  wall  not  reached. 

763.     10  ft.  east  of  762  in  26-ft.  level — 16  in.  of  quartz  on  footwall. 

Hi. — 20  in.   of  gossany  ore,   containing  considerable   manganese. 

Hs. — 21  in.  of  quartz  ore,  with  small  amount  manganese. 

HZ. — 36-in.  gossany  quartz,  with  manganese  and  clay.  Hanging  wall  not 
reached. 


'Name  given  to   workings  011   account   of  their  appearance   on  mine  plan. 


66  MINE    SAMPLING   AND   VALUING 

764.  10  ft.  east  of  763 — 34  in.  of  soft  gossany  ore,  containing  manganese, 
clay  and  quartz,  taken  to  back  of  drive.  Footwall  not  reached. 

764!!. — 39  in.,  same  ore,  across  back  of  drive.     Hanging  wall  not  reached. 

76$F. — 10  ft.  east  of  764,  15  in.  of  footwall  stuff,  apparently  mostly  country. 

765. — 26  in.  of  hard  gossany  white  quartz. 

765/7. — 42  in.  of  mixed  gossany  ore  (quartz,  clay,  and  manganese,  with 
nodules  of  country).  On  hanging  side  6-in.  manganese.  Hanging  wall  not 
reached. 

Resamples  for  Checking  Results. 

I38R.  10  ft.  east  of  137— Footwall  here  is  of  red  rock.  36  in.  of  hard 
gossany  quartz. 

I38RH. — 60  in.  of  soft  ore  containing  some  quartz,  and  considerable  man- 
ganese. Hanging  wall  getting  quite  smooth,  clayey  material  next  it. 

i4oR.  10  ft.  east  of  139,  opposite  cross-cut  990  East — 44  in.  of  hard  gossany 
quartz,  measured  from  retaining  wall.  Footwall  not  reached. 

I40RH. — 25-in.  soft  ore  containing  some  manganese,  and  a  little  hard  quartz. 

I42R. — 24  in.  of  soft  gossany  ore  with  manganese  on  hanging  side.  Below 
this  sample  on  foot  is  altered  decomposed  country,  containing  some  quartz,  which 
was  not  sampled. 

I44R.  8  ft.  east  of  143R  in  the  timbers — 39  in.  of  gossany  ore  containing 
some  quartz,  but  mostly  clay  and  manganese  ore,  to  hanging  wall. 

It  will  be  noted  that  i-n  the  resamples  quoted  only  alternate 
numbers  are  given,  the  odd  numbered  samples  were  taken  by  one  of 
my  assistants.  This  is  to  emphasize  the  advisability  of  having  two 
men  sample  in  company,  each  taking  alternate  intervals.  In  this 
way  if  there  is  any  error  due  to  personal  equation  it  will  be  dis- 
covered; besides,  if  the  responsible  engineer  be  one  of  the  two,  he 
is  enabled  personally  to  direct  where  and  how  the  samples  are  to 
be  taken  and  incidentally  watch  his  assistant's  work.  In  resampling 
it  is,  of  course,  not  necessary  to  describe  the  sample  fully  as  this  has 
already  been  done.  Resamples  should  generally  be  taken  along1  the 
same  cut  as  the  original. 

The  descriptions,  such  as  here  given,  may  mean  nothing  to  one 
unfamiliar  with  the  workings,  but  are  intended  as  a  guide  to  the 
engineer  himself  in  case  of  unexpected  results.  With  copper  or 
lead  mines,  it  is  useful  to  make  a  guess  at  the  assay  value.  A  small 
lump  of  chalcocite  placed  in  a  sack  of  low-grade  copper  ore  by  an 
interested  party  will  materially  raise  the  grade.  If  the  value  is 
estimated  and  set  down  in  the  notebook,  the  salting  may  be  de- 
tected. 

The  sample  number  should  be  plainly  noted  in  the  margin  of 
the  page,  followed  by  its  distance  from  the  last  sample,  or  the 
nearest  survey  station.  In  addition  it  should,  if  possible,  be  tied 
to  any  of  the  other  mine  workings,  such  as  a  cross-cut,  rise,  or 
winze.  This  facilitates  locating  the  sample  on  the  mine  plan  after- 
wards. The  width  sampled  should  be  measured  and  noted,  fol- 
lowed by  a  description  of  the  character  of  material.  For  instance, 
if  a  high  result  was  obtained  from  a  sample  containing  much 
country  rock,  it  may  be  taken  for  granted  the  result  is  incorrect. 

This  procedure  serves  the  purpose  not  only  of  impressing  on 
the  engineer's  mind  the  characteristics  of  the  orebody,  but  may 
enable  him  to  trace  causes  of  erratic  assay  values,  as  well  as 
indicating  in  what  class  of  material  the  values  are  contained. 


HANDLING  OF  SAMPLES  67 

Sacking  the  Sample. — Assuming  that  the  cuttings  of  ore  com- 
prising the  sample  are  in  a  sample  bucket,  it  becomes  necessary 
to  transfer  the  sample  to  one  of  the  small  bags  provided  for  the 
purpose.  It  has  been  recommended  that  two  sizes  of  bags  should 
be  provided  and  it  is  strongly  urg'ed  that  a  bag  large  enough  to 
take  the  entire  sample  be  used.  Dividing  the  sample  into  two  or 
more  bags  tends  to  error  in  the  assay  office. 

Where  a  gold  mine  is  being  sampled,  the  sample  sacks  should 
be  turned  inside  out  and  beaten,  in  order  to  shake  out  any  grains 
of  gold  dust  that  may  'inadvertently'  have  been  put  there  by  in- 
terested parties.  Before  the  sample  is  put  in  the  sack,  it  must  be 
again  turned,  so  that  the  seam  is  inside.  In  order  to  transfer  the 
sample  to  the  sack,  one  of  the  smaller  sheets  of  canvas  is  laid  on 
the  ground  and  by  means  of  the  hands,  or  by  pouring  from  the 
bucket,  the  sack  is  filled.  Care  must  be  taken  that  none  of  the 
particles  are  lost. 

Brittle  minerals  generally  contain  the  metals  of  highest  value 
and  because  of  their  brittleness  powder  easily.  For  that  reason, 
the  last  portion  of  the  dust  remaining  in  the  bucket  or  what  may 
overflow  onto  the  canvas,  must  be  carefully  gathered  and  added 
to  the  sample.  The  inside  of  the  bucket  should  be  beaten  with 
the  hand  or  other  object  to  get  out  these  last  portions. 

Some  engineers  may  say  that  the  discarding  of  the  dust  acts 
as  a  factor  of  safety,  but  this  is  bad  practice.  A  loss  of  any 
portion  of  the  sample  renders  the  result  inaccurate  and  if  the 
engineer  desires  to  provide  a  factor  of  safety,  he  should  do  so  with 
his  eyes  open  and  not  leave  it  to  chance. 

The  ore  being  in  the  sack,  its  number  should  be  marked  on  one 
of  the  small  linen  baggage  labels  with  an  indelible  pencil.  If  there 
are  two  sacks  composing  one  sample,  the  two  labels  could  be 
written  out  together  and,  to  avoid  error  in  the  assay  office,  each 
one  is  marked  'Two  Sacks/  and  the  two  sacks  tied  together  after 
they  are  sealed. 

Tying  and  Sealing. — This  sounds  like  a  comparatively  unim- 
portant operation,  yet  there  are  a  few  precautions  which  it  is 
advisable  to  indicate,  the  neglect  of  which  may  in  some  instances 
be  the  cause  of  erroneous  results. 

The  ore  should  be  well  shaken  down  in  the  sack,  so  that  it 
will  occupy  the  minimum  space,  and  the  linen  label  placed  on  top. 
The  mouth  of  the  bag  is  then  firmly  held  together  and  the  string 
tied  below  the  hand  as  close  to  the  ore  as  possible.  This  opera- 
tion can  better  be  done  by  two  people  than  one.  The  sack  should 
be  held  facing  the  person  tying  it,  and  taking  a  double  turn  of 
the  string  around  the  neck,  a  single  knot  is  tied  on  the  farther  side 
of  the  sack  and  drawn  tightly  together.  The  string  is  then  passed 
around  to  the  front  and  a  firm  double  knot  tied  as  near  the  centre 
of  the  sack  as  possible,  care  being  used  not  to  tie  a  granny's  knot. 
The  object  of  the  first  knot  is  to  prevent  the  string  loosening 
while  tying  the  second  knot ;  besides,  it  is  essential  that  the  mouth 
of  the  bag"  be  tightly  closed  to  prevent  any  possible  loss  of  fine 


68  MINE   SAMPLING   AND   VALUING 

material  during  transportation,  especially  when  samples  are  shipped 
long  distances  for  assay.  This  being  done,  the  sack  is  sealed  with 
a  clear  impression  of  the  seal  on  this  last  knot,  care  being  taken 
that  the  sack  is  not  burned  during  the  operation.  Some  engineers 
use  a  sack  already  supplied  with  strings,  which  are  sewn  on.  As 
these  are  always  near  the  mouth  of  the  sack,  when  the  sack  is  not 
full,  there  is  not  only  danger  that  the  loose  ore  will  so  mutilate 
the  label  as  to  make  the  number  illegible,  which,  of  course,  means 
a  resample,  but  tampering  with  the  sample  is  rendered  more  easy. 

It  is  very  difficult  to  tamper  with  the  sample  that  is  sacked 
and  tied  in  the  manner  described,  as  any  addition  to  the  ore  is 
impossible  without  showing  evidences  of  tampering.  Adding 
gold  dust  to  the  mouth  of  the  bag  is  a  hazardous  operation  as  the 
chances  are  that  none  of  the  gold  dust  will  ever  enter  the  sample, 
but  will  be  shaken  out  before  the  sack  is  opened  or  be  discovered 
in  the  assay  office. 

Of  course,  with  the  precautions  that  are  here  recommended  as 
to  the  safe-guarding  of  samples,  there  is  not  much  likelihood  of 
their  being  molested  by  others  after  they  are  once  sacked,  but  too 
many  safeguards  cannot  be  thrown  around  the  samples,  as  on  them 
the  whole  final  results  depend.  The  engineer  must  not  only  pro- 
tect the  interests  of  those  by  whom  he  is  employed,  but  he  must 
bear  in  mind  that  his  own  reputation  is  at  stake,  and  that  one 
serious  error  in  mine  valuing  is  apt  to  damn  him  forever.  One 
thing  that  financiers  cannot  forgive  in  an  engineer  is  the  loss  of 
money  through  that  engineer's  advice.  The  sample  once  sealed 
should  be  put  in  the  leather  mail  sack,  and  when  the  samples  are 
taken  from  the  mine,  if  it  is  not  done  before,  the  sack  should  be 
locked. 

Before  proceeding  to  take  the  next  sample,  sample  buckets 
that  have  been  used  should  be  turned  inside  out,  and  if  the  ore  be 
dry,  should  be  thoroughly  brushed  with  a  scrubbing  brush.  In 
wet  mines  the  scrubbing  brush  should  still  be  used,  but  with  the 
addition  of  water  to  wash  off  any  remaining  portion  of  the  previous 
sample.  The  canvas  sheets  used  should  also  be  thoroughly  cleaned 
before  using  again. 

A  distinctively  colored  string,  as  sometimes  recommended,  is 
no  additional  safeguard,  as  the  sealing  of  a  sack  effectually  pre- 
vents tampering  with  the  string,  besides  which,  colored  string1  is 
not  always  obtainable. 


CHAPTER  IX, 

PREPARATION  OF  SAMPLES  FOR  ASSAY. 

This  perhaps  belongs  more  in  the  realm  of  the  assayer  than 
the  engineer,  but  in  practically  all  cases  where  an  engineer  is 
called  upon  to  conduct  a  mining  examination,  he  must  watch  the 
sample  all  the  way  through,  even  to  the  assaying.  It  would  be 
useless  to  adopt  all  the  precautions  one  usually  does  in  taking  the 
sample  and  the  prevention  of  salting  underground,  if  in  the  end 
the  sample  is  turned  over  to  some  irresponsible  person  for  crush- 
ing and  assaying.  It  is  very  easy  to  accurately  salt  a  sample,  if 
one  has  the  handling  of  it  in  the  assay  office,  because  there  weights 
can  be  accurately  determined  and  the  sample  scientifically  salted 
to  any  desired  extent.  When  an  operating  mine  with  a  com- 
plete assay  plant  is  being  examined,  this  problem  is  simplified, 
because  the  engineer  usually  is  given  possession  of  the  assay  office 
and  such  apparatus  as  it  may  contain. 

It  may  be  stated  without  cavil  that  there  are  comparatively 
few  mines  that  engineers  are  called  upon  to  inspect,  fitted  with 
crushing  apparatus  of  sufficient  capacity  to  permit  of  rapid  hand- 
ling of  the  large  number  of  samples  usually  taken.  It  then 
devolves  on  the  engineer  to  make  some  arrangement  to  facilitate  his 
work,  and  often  considerable  ingenuity  must  be  employed  to  prevent 
the  loss  of  an  excessive  amount  of  time.  This  may  be  illustrated  by 
one  or  two  instances  from  practical  experience. 

In  one  examination  where  over  1,000  samples  were  taken  by  an 
engineer  and  two  assistants,  the  option  period  was  limited  and  the 
assay  laboratory  was  fitted  with  two  small  hand  jaw  crushers,  one 
bucking  board,  and  one  pestle  and  mortar  as  the  sole  crushing  ap- 
pliances. Another  bucking  board  was  obtained  from  a  mine  nearly 
150  miles  away  by  road,  and  an  additional  pestle  and  mortar  secured 
in  the  neighborhood.  With  eight  native  laborers  in  this  crushing 
room,  working  long  hours,  about  30  samples  per  day  were  prepared 
for  assay,  the  engineer  himself,  or  one  of  his  assistants,  being  in 
constant  attendance  watching  the  operations  and  doing  all  the  quarter- 
ing. Fortunately  the  laboratory  was  provided  with  two  Jones  dividers, 
which  greatly  facilitated  the  reduction  of  the  samples. 

Some  years  ago  I  was  unexpectedly  called  upon  to  examine  an 
ancient  silver-lead  mine  in  Burma.  In  the  course  of  the  work  some 
500  samples  of  lead  slag  carrying  about  50%  lead  were  taken.  There 
were  no  crushing  appliances  whatsoever  on  the  property  and  it  was 
impossible  to  get  any  in  the  country.  The  only  thing  to  do  was  to 
make  the  best  of  the  conditions  that  existed  and  to  prepare  the  samples 
for  assay  by  hand.  These  were  obtained  from  test  pits  put  down  in 
slag  piles  and  some  of  them  weighed  as  much  as  200  Ib.  The  follow- 


70  MINE    SAMPLING   AND   VALUING 

ing  modus  operand*  was  employed :  Large  canvases  were  laid  on  the 
ground  and  as  many  coolies  as  possible  placed  around  the  canvas,  each 
provided  with  a  hammer  and  a  large  rock,  to  serve  as  a  mortar  block. 
The  lumps  of  slag  were  crushed  until  the  whole  sample  would  pass 
about  a  /4-in.  rnesh  screen.  It  was  then  passed  over  a  riffle  divider, 
with  which  I  had  fortunately  provided  myself.  This  divider  was  set 
up  over  an  empty  kerosene  box  and  supported  at  the  corners  by  means 
of  four  long  nails.  By  subsequent  recrushing  and  dividing,  the  sample 
was  brought  down  to  proper  weight  for  fine  crushing.  Two  gangs  of 
men  were  employed  in  charge  of  an  intelligent  Burman  under  my 
direction  and  considerable  speed  was  attained.  The  results  were 
accurate,  as  the  subsequent  smelting  operations  demonstrated. 

The  following  may  be  taken  as  a  general  method  of  preparing  mine 
samples  for  assay.  In  some  cases  the  ore  may  be  so  dry  as  not  to 
require  any  preliminary  drying.  On  the  other  hand,  accurate  results 
in  reducing  the  sample  cannot  be  obtained  if  the  ore  is  moist.  The 
first  step  then  is  to  dry  the  ore,  if  it  be  necessary.  Here  again  we 
meet  with  a  problem.  If  the  ore  has  a  tendency  to  be  sticky,  great 
difficulty  may  be  experienced  in  thoroughly  drying  the  sample,  it 
occasionally  being  necessary  to  leave  such  a  sample  on  a  hot  plate  for 
six  or  eight  hours,  or  more.  In  the  first  illustration  quoted  in  this 
chapter,  the  ore  was  clayey  and  wet.  Fortunately  at  a  previous  exam- 
ination of  the  mine,  a  hot  plate  had  been  provided  by  using  a  sheet 
of  /4~m-  iron,  set  upon  brick  walls  fitted  with  a  door  for  firing  in  front 
and  connected  with  a  chimney,  so  that  a  hot  fire  could  be  maintained. 

No  doubt  theory  demands  that  in  order  to  secure  accurate  results, 
moisture  should  be  evaporated  from  a  sample  at  a  temperature  only 
slightly  above  the  boiling  point  of  water.  Had  such  a  procedure  been 
adopted  in  this  case,  it  would  have  taken  months  to  dry  the  samples. 
As  a  matter  of  practice,  it  was  found  that  the  only  way  to  get  results 
from  some  of  the  samples  was  to  have  the  plate  heated  to  redness. 
The  water  could  scarcely  be  driven  out  of  the  clay,  except  by  making 
brick  of  it.  It  can  be  readily  understood,  that  unless  the  clay  had  been 
heated  to  a  sufficient  extent  to  make  it  hard,  it  would  not  have  gone 
through  the  crushers  and  any  accurate  division  of  the  sample  would 
have  been  impossible.  This  is  cited  as  a  typical  case  where  the  en- 
gineer must  use  his  judgment  and  not  blindly  follow  theory.  The 
objection  to  heating  a  sample  to  the  extent  stated  can  only  be  that 
some  of  the  volatile  elements  will  be  driven  off  and  perhaps  with  an 
attendant  loss  of  some  of  the  precious  metals.  But  this  is  inevitable 
and  if  such  a  thing  were  to  occur,  the  error  could  only  be  on  the  side 
of  safety.  This  sort  of  factor  has  been  deprecated  in  these  pages, 
but  in  this  case  there  is  no  remedy. 

Russian  iron  pans  were  used  to  hold  the  sample  while  drying, 
each  pan  having  the  sample  number  marked  on  it  with  chalk.  If 
there  are  two  or  more  pans  for  one  sample,  it  should  be  so  noted 
on  each  of  the  pans.  The  original  labels  found  in  the  sacks  are  put 
on  one  side  and  at  the  end  of  the  day  checked  up  with  the  finished 
samples,  in  order  to  immediately  trace  any  errors.  Mistakes  are 
bound  to  happen  when  one  man  has  to  follow  a  number  of  samples 


PREPARATION  OF  SAMPLES  71 

through  the  various  operations  that  are  carried  out  by  unintelligent 
labor.  When  the  sample  is  dried,  it  is  put  on  one  side  to  cool  and 
relabeled  by  a  slip  of  paper  bearing  its  number,  preparatory  to 
carrying  it  through  the  next  series  of  operations. 

Occasionally  erratic  results  are  quoted  by  engineers,  but  these 
are  often  due  to  mistakes.  A  case  discussed  in  Mr.  Rickard's  book* 
showed  that  two  halves  of  an  original  sample  gave  widely  varying 
results,  because  the  reduction  of  a  wet,  sticky  sample  was  under- 
taken underground,  without  any  drying.  The  sample  contained 
wire  gold  and  was  mixed  on  a  canvas.  Accurate  results  could  have 
been  obtained  only  by  accident.  A  sample  containing  any  clayey 
material  should  be  thoroughly  dried  before  any  division  is  attempted. 
A  sample  of  low-grade  quartz  may  perhaps  be  divided  wet,  but  if  there 
are  any  valuable  sulphides  or  metallics,  even  with  a  clean  quartz 
gangue,  the  sample  should  be  dried  first. 

Where  proper  mechanical  crushing  appliances  are  available,  a 
sample  can  be  pulped  in  a  very  few  minutes.  Various  types  of  crush- 
ing apparatus  are  on  the  market,  some  better  than  others,  but  all  more 
or  less  well  known.  The  selection  of  such  apparatus  rarely  falls  to  the 
examining  engineer  and,  therefore,  can  be  dismissed  without  much 
comment. 

Occasionally,  however,  in  going  to  foreign  parts,  the  examining 
engineer  must  provide  himself  with  portable  outfit.  Of  the  hand 
crushers,  the  one  with  fly-wheels  is  more  efficient  and  preferable  to 
the  lever  type.  A  mechanical  grinding  machine  operated  by  hand  is 
a  useful  adjunct.  It  is  a  great  time  saver  over  the  pestle  and  mortar, 
as  jaw  crushers  cannot  be  depended  on  for  fine  crushing  and  there 
must  be  some  intermediate  apparatus  between  this  and  the  bucking 
board.  Only  one  caution  is  necessary  in  the  selection  of  apparatus, 
and  that  is,  care  must  be  exercised  that  the  machine  be  strong,  as 
near  fool-proof  as  possible,  and  that  parts  are  easily  accessible  for 
cleaning. 

After  the  sample  has  been  thoroughly  dried  and  relabeled  it  must 
be  crushed  to  small  size,  before  being  divided.  In  the  instances  previ- 
ously cited,  where  there  were  two  small  jaw  crushers,  one  of  them 
was  set  for  coarse  crushing  and  the  second  for  fine.  In  the  latter  the 
ore  was  reduced  to  pass  a  quarter-inch  screen,  although  nearly  all  was 
much  finer.  It  is  well  known  that  if  material  is  crushed  to  pass  a 
given  mesh,  then  probably  90%  would  be  much  finer,  so  that  a  great 
deal  of  time  can  be  saved  by  crushing  to  a  coarser  mesh  than  desired, 
screening  out  the  oversize  and  recrushing.  The  fineness  to  which  a 
sample  must  be  crushed  for  division  depends  altogether  upon  the  bulk 
of  the  sample.  A  number  of  engineers  have  contributed  to  our  knowl- 
edge of  the  subject  and  these  articles  are  easily  accessible.  A  paper 
by  D.  W.  Brunton,  published  in  Vol.  49  of  the  Transactions  of  the 
American  Institute  of  Mining  Engineers,  is  a  classic  and  should  be 
read  by  every  mining  engineer.  Some  of  the  works  on  assaying  also 
treat  of  this  subject. 

^'Sampling   and    Estimation    of    Ore,'    page    151    et    seq. 


72  MINE    SAMPLING   AND   VALUING 

Speaking  generally,  the  fineness  to  which  the  ore  should  be  crushed 
before  further  division,  depends  to  a  considerable  extent  upon  the 
character  of  the  material  that  is  being  sampled.  The  same  rule  applies 
in  this  case,  as  does  to  sampling  the  ore  underground,  namely:  The 
more  homogeneous  the  material,  the  more  evenly  distributed  the  valu- 
able mineral,  the  lower  the  monetary  value  of  that  mineral,  then  the 
less  care  is  required.  On  the  other  hand,  in  the  case  of  irregularly 
distributed  gold  with  high-grade  brittle  sulphides  or  free  gold,  great 
care  must  be  taken,  more  especially  if  the  ore  is  not  of  high  assay 
value,  because  a  small  proportion  of  the  valuable  mineral  going  into 
one  half  of  the  sample  or  the  other  may  seriously  affect  the  result. 
Where  coarse  gold  occurs  the  problem  becomes  exceedingly  difficult 
and  serious;  errors  are  liable  to  occur  and  the  greatest  care  must  be 
exercised  in  manipulating  such  samples  during  the  reduction  to  the 
final  pulp. 

It  will  be  seen  that  no  general  rule  can  be  laid  down.  The  engineer 
must  use  his  judgment  in  the  matter.  In  this  connection,  it  may  be 
pointed  out,  that  as  an  extra  precaution,  it  is  advisable  to  select  a  few 
samples,  carry  both  halves  of  the  original  sample  through  to  a  final 
pulp  and  assay  both  original  and  duplicate.  This  will  serve  as  a  check 
on  the  operations  and,  if  no  serious  differences  are  noted,  the  engineer 
will  have  the  assurance  of  knowing  that  the  work  has  been  properly 
done.  On  the  other  hand,  if  there  be  serious  differences  in  the  assay 
values  of  the  original  and  duplicate,  it  will  be  a  warning  to  revise  the 
method  employed. 

I  think  there  is  a  tendency  on  the  part  of  far  too  many  engineers 
to  make  a  division  of  the  original  sample  before  it  has  been  crushed 
to  a  sufficiently  small  size.  Probably  the  main  reason  for  this  is  the 
amount  of  time  and  labor  it  requires  to  do  this  work. 

Division  of  Sample. — After  the  sample  has  been  crushed  to  the 
proper  fineness,  it  must  be  divided,  if  a  mechanical  divider  is  at 
hand ;  if  not,  the  sample  must  be  quartered  in  the  ordinary  primitive 
way.  When  a  mechanical  divider  is  used  the  operator  must  see  that 
the  ore  is  distributed  evenly  over  the  scoop,  and  emptied  over  the 
divider  in  an  even  stream,  care  being  taken  not  to  fill  the  divider 
to  overflowing.  Sometimes  the  sample,  if  large,  can  be  divided  a 
second  time  without  further  crushing,  but  in  this  the  engineer  must 
be  guided  by  his  experience,  always  bearing  in  mind  that  the  greater 
the  fineness,  the  more  accurate  the  result.  In  some  cases  it  may  be 
advisable  to  crush  the  whole  remaining  material  to  pass  a  10-mesh 
screen  before  making  the  division  to  secure  the  weight  required  for 
the  final  pulp. 

A  discussion  of  the  methods  of  preparing  a  sample  for  assay 
will  not  be  complete  without  some  brief  reference  to  the  well  known 
method  of  quartering.  Of  course,  most  books  on  assaying  deal  with 
this  subject  more  or  less  in  extenso,  and  I  only  wish  to  briefly  refer 
to  it  to  round  up  the  present  discussion. 

An  engineer  may  find  himself  in  the  field  without  having  pro- 
vided mechanical  dividers  of  any  kind,  yet  large  samples  must  be 
reduced  in  bulk  in  order  to  permit  of  economical  transportation. 


PREPARATION  OF  SAMPLES  73 

Without  mechanical  dividers,  the  only  thing  that  is  left  to  be  done 
is  to  quarter  the  sample  on  a  canvas  sheet  or  on  an  ordinary  oil 
cloth,  what  is  known  in  England  as  American  cloth,  which  can  be 
bought  at  a  general  merchandise  store,  practically  in  any  part  of 
the  world.  The  only  other  accessories  that  are  required  are  a  stiff 
brush,  such  as  a  scrub  brush,  whisk  broom  or  horse  brush,  and  the 
usual  implements  that  are  found  on  any  mine. 

The  theory  underlying  quartering  is  the  same  as  that  underlying 
any  other  way  of  dividing  a  sample,  namely,  to  take  a  fraction  of 
the  total  bulk  and  have  such  fraction  represent  the  original,  i.e., 
have  the  same  assay  value.  The  method  of  procedure  is  as  follows : 

The  ore  is  crushed  to  a  sufficient  degree  of  fineness,  the  size  of 
the  largest  particles  depending  on  the  size  of  the  sample  and  the 
character  of  the  mineralization.  This  crushing  down  is  done  with 
a  hammer,  using  a  worn-out  mortar  block  of  a  stamp  battery,  any 
heavy  piece  of  iron,  or  even  a  large  stone  as  an  anvil  on  which  to 
break  the  lumps  of  ore.  When  the  ore  has  been  crushed  sufficiently 
fine,  it  is  thoroughly  mixed  on  a  canvas  sheet.  This  operation  is  a 
simple  one,  if  the  proper  method  be  followed.  What  is  required 
is  a  rolling  motion,  so  as  to  get  a  thorough  mixture.  If  the  corner 
of  the  canvas  is  simply  lifted,  the  ore  may  slide  down  the 
inclined  surface  and  no  mixing  take  place.  With  a  large  sheet, 
one  end  is  allowed  to  remain  on  the  ground ;  one  corner  of  the 
opposite  end  i-s  then  taken  and  by  a  twisting  motion  the  ore  rolled 
over  and  over  on  the  sheet,  so  that  the  desired  mixing  effect  takes 
place.  The  same  thing  is  done  alternately  from  each  corner  and  the 
same  action  is  repeated  from  the  opposite  end  when  the  ore  has 
rolled  up  far  enough.  When  the  ore  has  been  sufficiently  mixed  it 
is  coned  on  the  canvas  sheet,  that  is,  a  small  quantity  is  poured  into 
a  conical  heap  in  the  centre  and  successive  amounts  added  to  it, 
care  being  exercised  that  the  stream  of  ore,  as  it  leaves  the  shovel 
or  scoop,  falls  as  near  as  possible  onto  the  point  of  the  cone,  the 
intention  being  to  distribute  the  flowing  stream  of  ore  equally  on 
all  sides.  This  cone  is  flattened  by  spreading  the  ore  from  the 
centre  to  the  outer  edges  again,  using  a  circular  movement  with  a 
view  of  continuing  the  mixing.  For  spreading  the  ore,  a  shovel  or 
scoop  is  used.  The  shovel  is  held  upright  and  by  a  series  of  vertical 
reciprocating  movements,  at  the  same  time  moving  the  shovel  in  a 
circular  path,  the  cone  is  flattened  down.  The  top  is  smoothed 
off,  working*  from  the  centre  towards  the  edges  and  then  divided 
as  nearly  as  possible  into  four  equal  quarters  by  means  of  a  flat 
strip  of  wood  or  a  shovel.  Two  opposite  quarters  are  rejected.  The 
ore  within  the  two  diameters  marking  them  is  carefully  removed 
and  the  dust  belonging  to  these  quarters  is  carefully  gathered  with 
a  brush  or  small  broom  and  also  rejected.  With  brittle  sulphides 
the  dust  may  show  the  higher  assay  values  and  consequently  an 
undue  proportion  must  not  be  allowed  to  remain  with  the  part  of 
the  sample  which  is  retained,  as  the  result  may  thereby  be  vitiated. 
Successive  quarterings  are  carried  out  with  crushing  at  each  stage, 


74  MINE    SAMPLING    AND    VALUING 

dependent  on  the  size  of  the  sample,  until  it  is  small  enough  for  the 
assay  laboratory. 

Fineness  of  Crushing. — The  fineness  to  which  ore  must  be 
crushed  depends  very  largely  on  the  character  of  the  minerals  and 
the  value  of  the  richest  mineral  in  the  ore. 

A  valuable  paper  by  D.  W.  Brunton  on  this  phase  of  the  subject 
appears  in  Vol.  25  of  the  Transactions  of  the  American  Institute  of 
Mining  Engineers.  Mr.  Brunton  describes  a  series  of  experiments 
which  were  undertaken  to  determine  the  fineness  to  which  crushing 
must  be  carried  in  sampling'  gold  and  silver  ores  in  order  to  obtain 
results  within  an  allowable  limit  of  error. 

In  a  resume  of  Mr.  Brunton's  calculations,  which  will  serve  a 
useful  purpose  if  they  do  no  more  than  call  attention  to  the  im- 
portance of  this  subject,  he  states : 

"The  experiments  and  calculations  described,  and  a  general  con- 
sideration of  the  subject  indicate  that  the  size  to  which  ore  must 
be  crushed  for  sampling  in  order  to  come  within  an  allowable  limit 
of  error  will  depend  upon  :  (1)  The  weight  or  bulk  which  the  sample 
is  to  have.  Evidently  the  smaller  the  sample  the  finer  the  material 
must  be  crushed.  (2)  The  relative  proportion  between  the  value 
of  the  richest  mineral  and  the  average  value  of  the  ore.  If  the  aver- 
age grade  of  the  ore  is  high,  in  comparison  with  the  grade  of  the 
richest  mineral,  a  particle  of  richest  mineral  of  a  given  size  and 
value  will  have  less  percentage  effect  on  the  sample  than  the  same 
particle  would  have  on  the  same  amount  of  lower  grade  ore ;  there- 
fore, other  conditions  being  the  same  with  high-grade  ores,  we 
may  crush  more  coarsely  than  with  low-grade  ones,  and  still  keep 
within  the  same  percentage  of  error ;  while  if  the  richest  mineral  is 
of  comparatively  high  grade  a  particle  of  it  of  given  size  will  have 
a  greater  effect  on  the  sample  than  if  it  is  of  low  grade,  and  this 
will  necessitate  finer  crushing.  (3)  The  specific  gravity  of  the 
richest  mineral.  The  higher  the  specific  gravity  of  the  richest 
mineral,  the  greater  the  value  contained  in  a  particle  of  given  size 
and  grade,  and  hence  the  greater  the  influence  of  such  particle  on 
the  sample ;  from  which  follows  the  necessity  of  keeping1  down  the 
size  of  the  largest  particles  by  finer  crushing  than  is  required  when 
the  richest  mineral  is  of  lower  specific  gravity.  (4)  The  number  of 
particles  of  richest  mineral  which  are  likely  to  be  in  excess  or 
deficit  in  the  sample  is  evidently  an  important  factor;  a  liability  to  a 
large  number  necessitating  especially  fine  crushing.  But  such 
liability  can  result  only  from  imperfect  mixing,  and,  for  material 
mixed  with  average  thoroughness,  this  number  must  be  small. 

"The  relative  proportion  by  weight  or  bulk  of  the  richest  mineral 
and  the  low-grade  or  average  ore  will  not  of  itself  affect  the  size 

to  which  the  ore  must  be  crushed assuming  a  lot  of  ore 

properly  crushed  and  mixed,  the  principal  effect  which  a  large  pro- 
portion of  richest  mineral  has  is  to  increase  the  proportion  of 
maximum-sized  particles  of  richest  mineral,  with  reference  to  the 
whole  number  of  particles  composing1  the  lot,  or,  in  other  words, 


PREPARATION  OF  SAMPLES  75 

to  increase  the  probability  of  the  occurrence  of  maximum  variations. 
But  the  limit  of  the  magnitude  of  these  variations  is  just  the  same 
as  when  only  a  small  proportion  of  the  richest  material  is  present. 

"Inasmuch  as  the  effect  of  a  particle  in  excess  or  deficit  in  a 
sample  depends  only  on  its  weight  and  value,  as  compared  with 
those  of  the  sample  itself,  it  is  evident  that  the  size  or  weight  of 
the  lot  of  ore  from  which  a  sample  is  taken,  or,  in  other  words, 
the  proportion  of  the  lot  of  ore  taken  as  a  sample,  does  not  directly 
enter  into  the  question  of  determining  the  maximum  size  which 
the  ore  may  have. 

"The  results  of  the  investigations  recorded  in  this  paper  show 
how  absolutely  necessary  it  is  that  ore  samples  should  be  re- 
crushed  after  each  successive  'cutting  down,'  so  that,  as  the 
sample  diminishes  in  weight,  there  may  be  a  nearly  constant  ratio 
between  the  weight  of  the  sample  and  that  of  the  largest  particle 
of  ore  contained  therein." 

These  quotations,  above  all  else,  indicate  the  necessity  of 
crushing  the  ore  as  much  as  possible  before  any  division  of  the 
sample  takes  place. 

Preparation  of  the  Pulp. — Ordinarily,  only  a  bucking1  board  is 
available  for  grinding,  although  there  are  machines  that  will  grind 
ore  to  100  mesh  with  great  facility.  A  bucking  board  and  elbow 
grease  are  what  most  engineers  encounter. 

All  books  on  assaying  point  out  the  necessity  of  passing  every 
portion  of  the  pulp  through  the  screen  and  carefully  inspecting  the 
final  portion  for  the  detection  of  metallic  particles  and  state  that 
the  metallics  should  be  collected,  weighed,  and  assayed  separately. 
Where  the  bucking  is  done  by  an  intelligent  person,  there  is  greater 
security  that  this  final  operation  will  be  carried  through  in  the  man- 
ner that  it  should  be.  Where  one  is  depending  on  unskilled  or 
'native'  labor,  the  engineer  is  confronted  with  the  fact  that  these 
people  neither  appreciate  the  responsibility  connected  with  their  work, 
nor  do  they  understand  the  reasons  therefor,  and  unless  the  engi- 
neer is  particularly  careful,  he  may  find  either  that  the  last  portion 
of  the  sample  that  does  not  easily  go  through  the  screen  will  be 
thrown 'a way  or  it  will  simply  be  added  to  the  pulp  without  being 
reduced  to  the  proper  mesh.  This,  needless  to  say,  is  bad  practice 
and  may  lead  to  erratic  assay  results.  In  one  case,  on  taking  a  day's 
work  and  rescreening  the  pulps,  it  was  found  that  about  25%  of 
them  contained  some  oversize,  representing  this  final  difficultly 
crushed  material. 

Many  people  recommend  grinding  all  samples  to  100  mesh.  In 
most  cases  a  coarser  mesh  may  be  employed  without  inaccuracy. 
Here  again  one  must  be  guided  by  the  character  of  the  material  that 
is  being  sampled ;  100  mesh  should  be  used  when  the  ore  is  a  high- 
grade  gold  ore.  Low-grade  copper  ore  carrying  2  to  4%  of  copper 
or  thereabouts  and  little  or  no  precious  metal  values  can  be  put 
through  even  60  or  70  mesh.  Speaking  generally,  it  will  be  found 
that  an  80-mesh  screen  will  answer  all  ordinary  circumstances.  It 


76  MINE    SAMPLING   AND   VALUING 

must  be  borne  in  mind  that  the  finer  the  mesh,  the  more  time  required 
to  buck  down  the  sample. 

Final  Sample. — When  the  pulp  has  been  crushed  to  a  sufficient 
degree  of  fineness,  it  should  be  put  in  a  sample  envelope  with  a 
flexible  metallic  strip,  which  is  used  for  securely  closing  the  envelope. 
These  envelopes  are  sold  by  all  American  dealers  in  assay  supplies, 
but  I  believe  are  not  manufactured  in  Great  Britain.  A  substitute 
can  be  obtained,  specially  made  by  a  stationer,  in  which  the  metallic 
strip  is  replaced  by  a  strip  of  paper,  but  this  is  not  altogether  secure. 
Canvas  bags  are  too  bulky  for  this  purpose  and  bags  of  muslin  or 
other  light  material  permit  of  some  of  the  finer  dust  sifting  out 
through  the  mesh  of  the  cloth.  Paper  envelopes  have  the  great 
advantage  not  only  that  they  are  perfectly  secure,  retaining  none  of 
the  pulp  when  emptied  prior  to  weighing  up  a  charge,  but  permit  of 
the  sample  number  being  plainly  marked  on  the  outside,  in  such  a 
manner  that  any  sample  can  be  easily  picked  out  when  required  for 
re-assay  or  other  purpose. 

In  addition  to  marking  the  envelope  on  the  outside,  the  slip  of 
paper  bearing  the  sample  number,  which  has  been  carried  through 
the  successive  crushing  operations,  should  be  placed  in  the  envelope 
to  make  assurance  doubly  sure.  It  is  perhaps  needless  to  point  out 
that  the  marks  on  the  envelope  and  the  paper  should  be  compared 
whenever  the  sample  is  used. 


CHAPTER  X. 

ASSAYING  OF  SAMPLES. 

I  seriously  hesitate  to  enter  upon  this  field,  as  I  do  not  profess  to 
be  expert  in  the  art,  but  my  excuse  for  contributing  a  few  remarks 
on  the  subject  is  that  in  talking  the  matter  over  with  a  number  of 
young  engineers.  I  find  there  seems  to  be  a  lack  of  information  re- 
garding certain  points,  a  knowledge  of  which  greatly  facilitates  the 
speed  with  which  this  portion  of  the  work  can  be  done.  No  doubt 
all  that  follows  can  be  found  in  text-books  on  the  subject,  although 
probably  not  in  the  form  here  set  out. 

The  speed  with  which  assaying  can  be  done  depends  on  the  system 
employed.  In  American  mines  and  smelting  works',  according  to  the 
usual  method,  neither  crucible  nor  other  object  used  in  assaying  is 
marked  with  a  distinctive  number.  The  samples  are  set  out  in  any 
desired  order  or  succession  and  their  numbers  noted  in  the  assay 
record  book  in  the  same  order.  They  are  then  taken  one  by  one  in 
succession  and  weighed  out  for  assay  and  the  same  order  maintained 
in  all  the  successive  operations.  Until  the  final  result  is  reached, 
there  is  no  way  to  distinguish  one  assay  from  another,  without  count- 
ing back. 

To  give  a  concrete  illustration,  let  us  assume  that  sample  numbers 
1  to  N  inclusive,  in  their  respective  envelopes,  are  placed  in  order 
alongside  the  pulp  balance  and  that  a  muffle  furnace,  by  far  the 
cleanest  and  most  rapid  method  for  fire  assaying,  is  to  be  employed. 
Further  that  this  murHe  will  take  nine  crucibles  at  a  charge.  Nine 
crucibles  on  a  board  covered  with  asbestos  board  and  shaped  as 
shown  in  Figure  15  are  placed  beside  the  assayer  and  so  arranged 
that  No.  1  crucible  to  take  No.  1  sample  is  in  the  upper  left-hand 
corner,  No.  2  to  the  right  of  it  and  so  on,  working  from  left  to  right, 
so  that  No.  9  is  the  last  crucible  in  the  lower  right-hand  corner. 
When  ready  for  charging  into  the  furnace  sample  No.  9,  the  crucible 
in  the  lower  right-hand  corner  should  be  placed  in  the  muffle  in  the 
back  left-hand  corner,  and  the  others  following,  placing  them  from 
left  to  right,  until  No.  1,  the  last  to  go  in,  is  in  the  front  right-hand 
corner.  See  Fig.  16. 

When  the  fusion  is  completed,  No.  1  sample,  which  is  in  the 
front  right-hand  corner,  is  taken  out  first  and  poured  into  a  mould, 
so  that  it  comes  in  the  upper  left-hand  corner.  Crucible  moulds 
usually  having  three  or  six  moulds  are  placed  side  by  side,  and 
the  succeeding  samples  are  poured  into  the  successive  holes  of 
the  moulds,  until  one  mould  is  filled,  when  the  next  one  is  started 
and  filled  in  a  similar  manner  (see  Fig.  17).  As  soon  as  the 
buttons  have  cooled  sufficiently,  each  should  be  separated  from 
the  slag,  cubed  in  the  ordinary  manner,  and  placed  in  a  button 


78 


MINE    SAMPLING   AND   VALUING 


tray.  This  is  made  of  sheet  iron,  divided  up  into  square  com- 
partments of  any  number,  25  is  a  convenient  size,  each  compart- 
ment being  perhaps  an  inch  and  a  half  square  (see  Fig.  18).  No. 
1  sample  again  comes  into  the  upper  left-hand  corner,  and  work- 
ing from  left  to  right  the  succeeding  ones  follow. 


Fig.   15.     CRUCIBLES  READY  FOR 
CHARGING  INTO  MUFFLE. 


Fig.    16.     CRUCIBLES    IN   MUFFLE. 


f 


Fig.    17.     METHOD   OF  POURING  FUSIONS. 


ASSAYING    OF    SAMPLES 


79 


When  it  comes  to  cupellation,  the  cupels  are  placed  on  the 
asbestos-covered  board,  which  has  been  previously  used  for  the 
crucibles  (see  Fig.  19),  with -No.  1  cupel  in  the  upper  left-hand 
corner  and  the  last  one  in  the  lower  right-hand  corner.  The  last 
sample  being  placed  first  in  the  muffle  (see  Fig.  20)  and  the  others 
following,  working  from  left  to  right,  until  No.  1  occupies  the 


Fig.    18.     LEAD   BUTTON   TRAY. 


front  right-hand  corner  of  the  muffle  in  the  same  manner  as  when 
charging  the  crucibles.  Taken  out  in  the  inverted  order,  No.  1 
again  occupies  its  place  in  the  upper  left-hand  corner  and  so  on. 
The  same  system  is  followed  with  the  gold  and  silver  beads,  and 
is  carried  right  through  the  parting  operations  and  annealing  of 
the  gold  and  final  weighing.  In  recording  the  results  in  the  assay 


_L 


OOOO0O 
000000 

000000 
000000 


Fig.    19.     CUPELS    READY   FOR 
CHARGING   INTO   MUFFLE. 


00000 

1000000 
GXD00© 


Fig     20.     CUPELS    IN    MUFFLE. 


80  MINE    SAMPLING   AND    VALUING 

record-book,  No.  1  result  is  put  down  opposite  the  first  number 
in  the  book,  whatever  its  actual  serial  number  may  be,  and  so  on 
in  succession. 

This  method  may  appear  complicated  at  first  glance  and  liable 
to  cause  error,  but  it  is  an  enormous  time  saver  and  if  the  work 
is  intelligently  carried  out,  there  is  less  liability  to  error  and  cer- 
tainly less  loss  of  time  than  when  every  crucible,  button,  cupel, 
and  annealing  cup  is  marked  with  its  distinctive  number. 

It  is  used  throughout  the  United  States  and  in  other  parts  of 
the  world  in  large  mines  and  works,  and  where  the  element  of  time  is 
so  important,  as  usually  is  the  case  in  mine  inspection,  this  fact 
should  be  a  sufficient  recommendation  for  its  use.  The  same 
system  of  placing  the  utensils  should  be  used  throughout  all  the 
operations.  With  two  men  in  an  assay  office  working1  together, 
anywhere  from  100  to  150  samples  may  be  in  process  and  where 
a  large  number  of  samples  have  been  taken,  it  is  practically  im- 
possible to  put  a  number  of  three  or  four  figures  on  a  crucible  with  any 
assurance  that  it  can  be  recognized  after  a  fusion  has  taken  place 
and  some  of  the  slag  crept  over  the  top  and  down  the  sides,  gloss- 
ing the  clay  and  thus  obliterating  the  number. 


PART  II. 
MINE  VALUING. 


CHAPTER  XL 

ESTIMATION  OF  ORE. 

Part  I  has  been  taken  up  with  a  discussion  of  the  methods  of 
sampling.  We  next  have  to  consider  the  interpretation  of  the 
assays  of  the  samples,  giving  due  weight  to  modifying  geological 
and  economic  conditions. 

It  may  be  taken  for  granted  that  the  engineer  is  by  this  time 
in  possession  of  a  plan  and  section  of  the  underground  workings. 
If  none  existed  previous  to  his  visit  to  the  property,  then  one  of 
his  first  duties  should  be  to  prepare  them,  not  only  for  his  guidance 
during  the  preliminary  inspection  and  subsequent  sampling  opera- 
tions, but  also  as  being  necessary  for  the  preparation  of  an  assay 
plan.  An  assay  plan  is  a  graphic  representation  of  the  manner 
in  which  the  valuable  minerals  occur  and  serves  as  a  guide  in 
determining  the  assay  value  and  tonnage  of  ore  in  sight. 

Faults,  intrusions,  changes  in  the  formation,  and  any  other 
geological  data  which  may  seem  to  have  any  bearing1,  should  be 
noted  on  the  mine  plans,  as  an  additional  aid  in  sizing  up  the  ore 
deposit.  It  may  be  stated  as  a  broad  fact,  that  the  same  class  of 
geological  phenomena  usually  occur  throughout  the  same  ore 
deposit ;  for  instance,  where  faulting  has  taken  place  it  will  gener- 
ally be  found  that  the  orebody  is  displaced  in  the  same  direction  in 
each  successive  instance,  so  that  if  a  mistake  has  been  made  in  the 
development  work  and  a  particular  face  has  passed  out  of  the 
ore  beyond  a  fault  plane,  the  location  of  these  faults  on  the  plans 
will  show  in  which  direction  a  continuation  of  the  orebody  may  be 
looked  for.  This  cannot  be  taken  as  a  hard  and  fast  rule,  because 
often  reverse  faults  exist  and  then  the  continuation  of  the  orebody 
must  be  looked  for  in  the  opposite  direction. 

Before  proceeding  to  plat  the  assay  results  on  the  mine  plan, 
the  engineer  must  do  a  considerable  amount  of  preliminary  work 
by  way  of  correlating  the  assays.  The  orebody  has  been  sampled 
in  fractions  and  intervals,  not  only  on  account  of  irregularity  in 
the  exposures  but  also  because  more  than  one  character  of  ore  may 
be  exposed  at  particular  intervals. 

Calculation  of  Averages. — The  calculations  should  be  carried 
out  in  a  book  or  books,  especially  ruled  off  for  that  purpose  and 
divided  into  sections  for  each  portion  of  the  mine;  that  is  to  say,  to 
correspond  to  each  serial  number.  These  books  may  be  labeled 
'Ore  Reserve  Calculations.'  I  find  a  journal  ruled  foolscap-size 
book  as  useful  as  any.  The  accompanying  diagram  shows  the  ruling 
and  headings — the  last  two  columns  are  obtained  from  calculations. 

The  sampling  record  from  the  underground  note  books  is  trans- 
ferred to  the  left-hand  page  and  the  results  of  the  assays  taken  from 
the  assay  office  records  noted  on  the  right-hand  page. 


84 


MINE    SAMPLING   AND   VALUING 


|    1 

£    ! 

W5 

°           VH            0 

vO                    JH                            0)  *^ 

ir 

00 

*4-(       (-( 

^i$  «2  .1 

d   *   J 

g    -S     1                o| 

ff  3*r 

I        M 

~^  i 

OO 

V    > 

c  6 
£  ^    j 

CN 

s 

T—  I 

•5^   | 

CO  c<> 

^ 

CN 

*"   a 

<2 

»                   vc 

"H 

J4-£<» 

t- 

f  £| 

T 

II 

T 

O   flj   C 

rO     D      C 

H     Q 

iO        TJH        O\     1    ,—. 

IO              TH              O              VO       1      _4 

IPO 

O        t-        t-        S 

\^            —  ^,            ^vq            ^vq      1     ^^ 

^N  p^ 

^  >  *7T 

*""*        OO        ON    1    ^"^ 

r^       ox 

**! 

g£u 

< 

°    s 

iO         ^^ 

O        t-        O 
IO         00         »O 

O        iO        O                         CO 
iO         CN         O                            y-< 

1y 

bo      " 

$1 

^        ON 

IO         TH         CN 

^=                      CN 

d      ^      od                  I-H 

CN 

||f 

< 

"*°.s' 

0         CN 

IO        iO        O 

IO        l>-        O 

•*    .  o 

£ 

•rf         CN         IO 

t^         CO         IO 

E'C  « 

«o 

^ 

T-H            TH 

aO         CN 

^H      d      d 

o  5^2 

CO 

. 

g|l 

2 

O            OO 

838 

_ 

iO        00        O                         fO 
t^       oo      10                    •«-! 

>    -^ 

^ 

"o 

vd        t^ 

iO        »-i        O 

OO        CO       t"»                         •«—  i 

T3  ^  « 

^                      CN 

a 

ill 

ill 

2        $        § 

O         CN         CN         "* 

ro        CN        co        CO 

O         O         CN         CN                O         CN 

ro       CN       CN       t-»             CN       ON 

'pl  *-;  j^ 

S"o  >^ 

sii 

en  -a  ^ 

CO 

|.E| 

CN 

DESCRIPTION 

3  ft.  west  of  survey  plug  No.  79  and  just  past  N. 
Crosscut  415  W.  Hard  white  quartz  on  foot- 
wall  along  side  of  drift  
Ferruginous  ore  in  back  of  drive,  contains  small 
amount  manganese  

10  East  of  261.  Soft  oxidized  ore,  containing  con- 
siderable iron  oxide,  and  a  little  clay  mixed  with 
gossany  quartz  on  footwall  side  of  drive.  Foot 
wall  not  reached  •  
Horse  of  mineralized  country  rock,  containing 
stringers  of  quartz  
Hard  quartzy  ore  showing  iron  stains  and  small 
amount  of  manganese.  To  hanging  wall  

At  N.  Crosscut  115E  samples  across  back  of  drift. 
Hard  quartzy  ore  on  hanging  containing  man- 
ganese streaks  and  some  iron  
Highly  altered  country  containing  numerous 
stringers  of  quartz  and  limonite  
Soft  ore,  manganese  banded  with  gossany  quartz 
and  clayey  material  

Barren  looking  quartz  on  foot  wall  

. 
• 
. 

c|.t: 

fa  (-i  p 

t)   .    *0 

.550 

"5    .  £*** 
.2  j^-  «j 

"  C       .5 
G         C   *ii 

|.S|| 

6 

TH         CN 

^H         CN                                     CO 

£ 

W 

pr  .           f-r  , 

ffi     ffi                   W 

T0^<J   o 

o 

T—  1                      ^-H 

CN                       CN         CN 

IO                IO         IO                                     tO 

(^   C,       ^^ 

TH 

*cL 

0              O 

CN                 fN          CN                                       CN 

S 

g 

T-H                    vH 

CN                       CN         CN 

%i-s 

CO 

•H 

*o  >  ^ 

ESTIMATION     OF     ORE  85 

In  case  the  ore  contains  base  metal  of  value,  appropriate  columns 
should  be  provided  for  noting  percentages  and  'inch  by  per  cent' 
(width  by  per  cent).  Sufficient  space  should  be  left  between  the 
successive  samples  to  note  the  results  of  any  re-samples  or  re-assays 
and  the  results  of  such  noted  in  red  ink. 

The  assay  results  once  transferred  to  this  book  from  the  assay 
office  record  permits  of  a  quick  determination  of  erratic  distribu- 
tion of  valuable  minerals.  If  any  occur,  a  re-assay  should  be  made 
of  the  particular  sample  in  question,  or  it  may  be  deemed  advis- 
able immediately  to  make  a  re-sample  of  the  exposure.  Those 
intervals  that  are  re-sampled  should  be  marked  with  the  same 
number  as  the  original  and  are  differentiated  from  it  by  adding 
the  letter  R  to  the  number;  thus,  if  the  original  number  is  147,  the 
re-sample  is  marked  itfR,  147  RFi,  147  RF2,  etc. 

The  calculation  of  the  assay  value  of  each  individual  interval  is 
done  by  combining  the  results  of  the  fractional  samples,  bearing  in 
mind  subsequent  mining  operations.  If  a  high-grade  streak  occurs 
on  one  of  the  walls,  it  may  be  necessary,  owing  to  the  low  assay  value 
of  the  remaining  portion  of  the  orebody,  to  consider  the  advisability 
of  stoping  the  high  grade  by  ifself.  One  cannot  arbitrarily  decide  on. 
such  a  procedure;  the  geological  and  mining  conditions  must  be  kept 
in  view,  as  it  may  not  be  economically  possible  to  mine  one  portion 
of  the  orebody  alone. 

These  questions  must  be  definitely  settled  at  the  time  of  calculating 
the  interval  assay  values  and  widths,  because  they  are  used  for  the 
assay  plan  and  for  calculating  the  average  assay  value  and  average 
width  of  the  whole  orebody,  and  thereby  enabling  a  determination 
of  the  total  metal  contents  of  the  ore  to  be  mined. 

In  averaging  up  fractional  samples,  profitable  ore  may  show  on 
either  wall,  with  unprofitable  ore  between.  If  the  low-grade  streak 
be  sufficiently  wide,  it  may  be  possible  and  economical  to  allow  it  to 
stand  as  a  pillar.  If,  however,  the  whole  width  of  the  orebody  must 
be  broken  in  the  subsequent  mining  operations,  the  low-grade  ore 
must  be  averaged  with  the  high,  in  order  to  ascertain  the  breaking 
value  of  the  ore  for  the  subsequent  calculations. 

Values  may  tail  off  towards  one  wall,  but  with  no  visible  geological 
change  in  the  formation ;  in  such  case,  the  engineer  must  use  his  knowl- 
edge of  the  orebody  and  his  mining  experience  to  determine  whether 
it  will  be  possible  to  break  the  ground  in  such  a  manner  as  to  leave 
the  unprofitable  ore  standing.  In  arriving  at  a  conclusion  he  may 
have  in  mind  subsequent  hand  sorting  to  raise  the  grade  of  the 
ore  after  it  is  broken,  but  before  it  is  sent  to  the  mill ;  or  he  may 
deem  it  possible  to  break  ore  to  a  required  grade  guided  by  careful 
stope  sampling.  In  either  instance  it  should  be  remembered  that  a 
greater  allowance  for  decrease  in  .grade  must  be  made  than  is  shown 
by  taking  the  proportionate  widths. 

Whether  sorting  be  adopted,  or  careful  stoping  methods,  a  con- 
siderable admixture  of  the  low-grade  material  is  inevitable  and  the 
assay  value  will  be  diminished  accordingly.  Under  such  conditions 
the  ore  cannot  be  mined  as  clean  as  it  can  be  sampled,  and  it  can  readily 


86  MINE    SAMPLING   AND   VALUING 

be  appreciated  that  no  sorting  operation  will  enable  the  whole  of  the 
poor  rock  to  be  picked  out. 

Where  the  orebody  is  narrow  and  some  of  the  wall  rock  must  be 
broken  to  get  a  stope  wide  enough  to  work  in,  care  should  be  taken  to 
make  ample  allowance  for  the  admixture  of  wall  rock.  In  the  narrow 
reefs  on  the  Rand,  it  is  customary  to  calculate  the  average  assay  value 
for  a  stoping  width  of  36  in.,  but  actual  practice  shows  that  only  about 
75%  of  the  calculated  grade  is  obtained.*  This  is  probably  due  to  the 
fact  that  unless  one  is  mining  to  a  slip  or  head,  no  accuracy  of 
dimension  can  be  attained  in  ordinary  mining  work. 

Diagram  I  illustrates  the  method  of  arriving  at  the  average  assay 
value  based  on  actual  practice  and  showing  three  different  conditions. 
The  figures  are  hypothetical.  The  calculation  of  the  average  of  two 
or  more  samples  is  based  on  the  premise  that  a  mixture  of  materials 
of  different  value  will  have  an  average  value  directly  proportional  to 
the  quantity  and  value  of  each  of  the  components. 

To  illustrate  this  by  a  simple  and  concrete  case,  let  us  assume : 

1  ton  of  ore  assaying  2      oz.  gold 
mixed  with  2  tons  "      "  "         1       "       " 

the  average  is  not 

(a)  3  tons  "     "  "         \y2  "      " 

but  is 

(b)  3  "   "    "  \y3  "    " 

(a)  corresponds  to  the  arithmetic  average,  while  (b)  represents  the 
volumetric  average  which  gives  the  correct  result.  The  arithmetic 
average  is  arrived  at  by  taking  the  sum  of  the  assays  and  dividing  by 
the  number  of  samples.  That  this  is  incorrect  can  be  seen  by  calculat- 
ing the  total  contents  as  follows : 

Tons  Assay  Total  contents 

1  X         2  oz.  2  oz. 

2  X         1  oz.  2  oz. 

Totals        3  tons  4  oz. 

If  3  tons  of  the  mixture  contains  a  total  of  4  oz.,  each  ton  will  have 
an  average  value  of: 

4  _,_  3  =  iy3  oz. 

It  is  obvious  that  the  richer  material  adds  a  greater  quantity  of  gold 
to  the  mixture  than  the  poor  stuff  and  the  arithmetic  average  takes  no 
account  of  this. 

Changing  the  above  proportions,  but  keeping  the  same  figures : 

2  tons  of  2  oz.  ore  —  4  oz. 

+   1      "      "1  oz.    "     =  1    " 

5  oz. 
giving  an  average  of  1  2/3  oz.  per  ton. 

The  averaging  of  mine  samples  is  governed  by  the  same  principle, 
but  instead  of  having  the  actual  quantities  to  deal  with,  they  are  repre- 

*E.    J.    Way,    Trans.    Inst.    Min.    and    Metallurgy. 


ESTIMATION     OF     ORE  87 

sented  by  samples  which  are  weighted  proportionally  to  the  tonnage 
of  ore  they  are  supposed  to  represent,  which  in  turn  is  directly  pro- 
portional to  the  width  sampled. 

In  the  illustration  just  given,  in  each  component  the  quantity  was 
multiplied  by  the  assay  which  gave  a  factor  in  this  case  the  total  gold 
which  added  together  with  the  other  factor  arrived  at  in  the  same 
way  and  divided  by  the  total  quantity  gave  the  average  assay  value. 

In  averaging  mine  samples,  as  the  tonnage  is  directly  proportional 
to  the  width  sampled,  the  width  is  used  instead  of  tonnage. 

The  mathematics  governing  this  method  is  fully  discussed  both  in 
H.  C.  Hoover's1  and  T.  A.  Rickard's2  books  and  no  good  purpose  is  to 
be  served  by  repeating  it  here.  That  the  arithmetrical  average  gives 
an  incorrect  result  has  been  demonstrated  by  the  two  problems  already 
given. 

To  get  the  volumetric  average,  the  following  is  the  general  method 
employed. 

The  assay  value  for  each  metal  is  multiplied  by  the  sample  width  in 
inches.  This  gives  an  arbitrary  factor  called  the  inch-shilling,  inch- 
dollar,  inch-pennyweight,  or  inch-per-cent,  as  the  case  may  be.  The 
sum  of  these  for  the  samples  to  be  averaged,  divided  by  the  sum  of 
the  widths  sampled,  gives  the  average  assay  value  of  the  lot.  Americans 
generally  measure  widths  in  feet  and  decimals,  in  which  case  the  foot- 
dollar,  or  foot-per  cent  is  used  as  a  factor. 

Referring  to  Diagram  i,  sample  No.  101  is  a  simple  case  where  the 
two  fractions  will  be  mined  together.  To  get  the  average,  multiply 
14  (width  of  101)  by  $7.50  (total  gold  and  silver  value)  which  gives 
105  inch-dollars,  to  which  must  be  added  46  (width  of  101  H)  multiplied 
by  $19.00  (total  gold  and  silver  value  of  same)  or  874  inch-dollars, 
giving  a  total  of  979  inch-dollars.  Divide  this  by  60,  the  total  width 
(the  sum  of  14+46),  and  there  results  a  quotient  of  163.27,  which  is 
the  average  value  of  the  whole  interval  for  the  total  width  of  60  inches. 

Looking  at  the  results  of  the  interval  262,  it  is  to  be  observed  that 
the  foot  and  hanging  wall  portion  are  pay  ore  while  the  central  fraction 
262F  is  below  grade.  As  the  whole  of  this  must  be  mined  together, 
the  low-grade  portion  must  be  averaged  with  the  other  two.  The  total 
inch-dollars  926  divided  by  the  sum  of  the  widths  84  gives  an  average 
assay  of  $11.03  for  the  interval  over  a  width  of  84  inches. 

Taking  Interval  725  next,  it  is  seen  that  on  the  footwall  (sample 
725  H3)  there  is  a  low-grade  band  of  hard  material  lying  adjacent  to 
soft  ore  (samples  725,  725H,  725H2)  and  that  in  stoping  operations 
this  band  of  hard  ore  can  be  left  standing.  Therefore,  as  it  will  not 
be  mined  it  need  not  be  included  in  calculating  the  average  value  of 
the  interval.  The  total  inch-dollars  for  the  three  fractions  averaged 
divided  by  the  sum  of  the  widths,  72  in.,  gives  an  average  of  $12.16  for 
a  total  width  of  72  in.  Of  course,  to  obtain  this  result  the  mine  man- 
agement would  have  to  work  the  mine  accordingly. 

On  the  other  hand,  it  is  obvious,  were  this  material  of  the  same 
physical  character  as  the  overlying  fraction  725  F2,  it  would  have  to 

^Principles   of   Mining.' 

2'Sampling   and   Estimation   of   Ore.' 


88  MINE    SAMPLING    AND   VALUING 

be  mined  and,  therefore,  averaged  in  the  result.  Likewise,  if  there 
were  a  band  of  soft  low-grade  ore  on  the  hanging  wall  side  of  the 
orebody  it  could  not  be  depended  on  to  stand  in  stoping  and  would 
have  to  be  averaged  in.  These  are  typical  problems,  but  each  case 
must  be  decided  on  its  merits  by  the  engineer  when  correlating  his  re- 
sults. 

Check-Sampling. — Before  proceeding1  with  the  calculations  to 
determine  general  averages*,  it  is  always  advisable  to  check-sample, 
say,  10%  of  all  the  samples  taken.  This  work  should  be  distributed 
over  various  parts  of  the  mine,  and  the  re-sample  should,  if  possible, 
be  taken  by  a  different  man  than  the  one  who  cut  the  original  sample. 
If  the  assays  of  the  re-sample  check  the  original  results  fairly  closely, 
say  within  10%,  it  may  be  assumed  that  the  sampling  and  assaying 
has  been  correctly  done  and  the  averaging  up  of  assays  in  the  dif- 
ferent blocks  of  ground  may  be  proceeded  .with. 

Erratic  High  Assays. — The  record  book  now  shows  the  aver- 
ages of  the  fractional  samples  for  each  different  interval.  These  must 
be  again  scanned  for  erratic  high  assays.  By  this  is  meant  the  occur- 
rence of  a  high  assay  amid  comparatively  lower  ones.  For  instance, 
if  the  general  run  of  assays  along  a  portion  of  a  particular  level  is 
in  the  neighborhood  of  $10  and  among  them  is  found  one  or  two 
showing  $25,  the  occurrence  is  erratic  and  the  intervals  should  be 
re-sampled.  This  occurrence  may  be  due  to  an  unobserved  high-grade 
bunch  or  to  the  accidental  inclusion  of  an  undue  proportion  of  a 
known  high-grade  streak  or  of  native  gold  in  the  sample. 

It  has  been  pointed  out  that  the  object  of  mine  sampling  is  to  de- 
termine the  stoping  value  of  the  ore,  so  that  if  in  a  group  of  ten 
successive  samples,  eight  showed  a  $10  assay  and  two  a  $25  assay, 
the  two  high  assays  would  raise  the  general  average  from  $10  to  $13, 
equal  to  $3  per  ton,  or  30%.  It  is  obvious  to  one  familiar  with  the 
geology  of  ore  deposits  that  the  amount  of  ore  corresponding  to  the 
portion  showing  this  high  value  is  insufficient  when  mixed  with  the 
remainder  to  raise  the  general  average  value  of  the  whole  quantity 
by  that  amount.  As  a  general  rule,  such  isolated  or  erratic  occurrences 
of  high-grade  material  may  be  expected  to  extend  upward  only  for 
about  the  same  distance  as  exposed  in  the  working,  but  whatever  the 
reason  may  be,  it  has  been  found  by  experience  that  the  only  safe 
method  of  procedure  is  to  cut  down  the  value  of  these  high-grade 
samples  to  the  general  average  of  the  others,  that  is,  to  $10  in  the 
particular  instance  given. 

,  In  a  paper  entitled  'Some  Sampling  Results'  by  the  late  E.  H. 
Garthwaite,  appearing  in  Vol.  XVI  of  the  Transactions  of  the  In- 
stitution of  Mining  and  Metallurgy,  some  startling  sampling  results 
are  shown,  because  the  erratic  high  assays  were  not  eliminated  in  the 
calculation  of  the  general  averages.  Four  examples  are  given,  and 
in  each  one,  the  samples  were  taken  at  2-ft.  intervals  and  all  the 
permutations  for  2,  4,  6,  8  and  10-ft.  intervals  worked  out,  that  is, 
2  for  4  ft.,  3  for  6  ft.,  4  for  8  ft.  and  5  for  10  ft.  Mr.  Garthwaite's 

*The  method  of  calculating  the  average  value  of  a  block  of  ground  is  discussed  later. 


ESTIMATION     OF     ORE  89 

paper  is  well  worthy  of  study  and  illustrates  in  an  illuminating  manner 
the  serious  error  that  may  arise  from  not  following  the  method  sug- 
gested above.  There  are  interesting  details  in  all  of  the  cases  which 
he  cites,  but  for  the  purpose  of  this  discussion  it  will  be  sufficient  to 
point  out  the  error  in  the  manner  of  deducing  the  average  result  in 
his  example  No.  4. 

We  find  that  62%,  or  35  samples,  of  the  total  number  taken,  assayed 
less  than  5  dwt.  of  gold  per  ton,  while  91^%,  or  52  samples,  assayed 
less  than  20  dwt.  of  gold  per  ton.  Of  the  balance  of  five  samples, 
representing  8^%  of  the  total 

1  assayed  between      20  and    30  dwt. 
1  "40    "       SO    " 

1  "          200    "     300    " 

2  300    "     400    " 

Mr.  Garthwaite  states  that  his  usual  practice  was  "to  reduce  any 
high  assays,  say  of  100  dwt.,  or  over,  to  the  average  of  two  samples 
above  and  two  below  and  (including  the  original)  take  the  average 
of  the  five  samples." 

Averaging  an  erratic  high  result  with  two  samples  above  and 
two  below,  does  not  meet  the  requirements  of  good  practice.  For 
reasons  stated  in  the  paper,  which  are  not  altogether  good,  he  omitted 
doing  even  this.  The  exact  assays  of  the  five  samples  quoted  are  not 
stated  in  Mr.  Garthwaite's  paper;  hence  it  is  impossible  to  calculate 
the  actual  effect  that  their  inclusion  had  on  the  remaining  52.  But 
it  may  be  laid  down  as  a  rule  to  be  followed  almost  invariably  in  cases 
of  this  sort,  that  every  one  of  these  five  samples  should  have  been 
eliminated,  or,  in  other  words,  reduced  to  the  general  average  of  the 
other  52.  A  study  of  the  complete  data,  which  is  not  available,  may 
even  reveal  that  some  of  the  remaining  sample  values  should  have  been 
reduced  as  well. 

The  paper  shows  the  variations  in  the  averages  for  the  different 
permutations  of  the  2-ft.  intervals,  but  those  obtained  in  the  different 
10-ft.  intervals  will  sufficiently  illustrate  the  point  at  issue.  They  are 
as  follows: 

Average  Average 

value  width 

dwt.  inches 

6.96  53.5 

2.01  51.0 

38.38  480 

3.70  50.0 

32.52  51.0 

The  average  of  the  2-ft.  intervals  gave — 
17.47  dwt.  over  49.0  inches. 

As  compared  to  these  results  of  Mr.  Garthwaite  samples  at  10 
ft.  intervals,  taken  by  the  mine  staff,  gave  6.83  dwt.  over  51.5 
inches  and  an  independent  sampling  gave  5.41  dwt.  over  53  inches. 

To  show  the  effect  that  the  inclusion  of  these  five  high  assays 
had  on  the  general  average  of  the  lot  let  us  work  out  their  volumet- 


90  MINE    SAMPLING   AND   VALUING 

ric  average,  taking  the  lower  figures  of  the  limits  given  and  assum- 
ing equal  widths,  as  the  exact  widths  are  not  available. 

Thus:  Sample  Assay  Quantity   X   Value 

1  X  20  dwt.  20 

1  X  40    "  40 

1  X  200    "  200 

2  X  300    "  600 

Totals        5  860 

This  figure  860  (being  the  sum  of  the  quantity  X  value  factors) 
when  divided  by  57  (the  total  number  of  samples  taken)  gives  a 
quotient  of  15 — the  number  of  pennyweight  by  which  the  average  assay 
has  been  increased  on  account  of  the  inclusion  of  the  five  samples. 
Of  course,  this  is  not  quite  accurate  as  no  account  has  been  taken  of 
exact  widths,  but  it  does  serve  very  plainly  to  indicate  the  large  error 
that  resulted  from  the  inclusion  of  these  erratic  assays.  Of  all  the 
samples  62%  averaged  less  than  5  dwt.  and,  in  order  to  increase 
the  general  average  from  5  to  l7J/2  dwt.  (Garthwaite's  average  for 
2-ft.  intervals),  equal  to  250  %,  the  remainder  of  the  samples  must 
represent  a  much  larger  tonnage  of  high-grade  ore  than  obviously 
they  did. 

In  the  paper  there  is  no  information  which  will  enable  one  to 
account  for  these  erratic  high  assays.  They  may  have  been  due  to 
the  inclusion  of  an  excessive  amount  of  an  easily  sampled  high- 
grade  streak  in  the  orebody  or  to  the  presence  of  coarse  gold.  As 
the  independent  sampling  and  the  mine  sampling  show  materially 
lower  results,  we  must  assume  that  in  these  last  two  samplings 
the  high  grade  could  not  have  played  such  an  important  part,  either 
the  sampling  itself  was  more  carefully  done  or  the  results  inter- 
preted more  conservatively. 

That  the  lower  average  value  obtained  by  them  corresponds 
more  nearly  to  the  breaking  value  of  the  ore,  there  can  be  little 
doubt,  although  sampling  at  2-ft.  intervals  ought  to  give  more 
accurate  results  than  at  10-ft.  intervals.  Perhaps  the  moral  to  be 
drawn  from  this  is,  that  excessive  refinement  in  work  of  this  kind 
is  not  always  worth  the  effort,  especially  when  omitting  to  use 
correct  methods  in  part  of  the  operation. 

There  are  exceptions  to  every  rule,  and  in  some  orebodies  there 
is  a  persistent  recurrence  of  erratic  high  assays,  which,  if  excluded 
from  the  average,  would  erroneously  show  unprofitable  ore.  A 
case  of  this  kind  cited  by  Walter  McDermott  in  the  discussion  of 
Garthwaite's  paper  is  that  of  the  Tomboy  mine  in  Colorado.  This 
property  has  been  a  consistent  dividend  payer  for  a  number  of 
years.  The  general  mass  of  the  orebody  is  of  low  grade,  but  there 
is  a  persistent  recurrence  of  small  bunches  of  high-grade  ore, 
which  of  course  show  erratic  results  in  sampling,  but  these  must 
be  averaged  in  with  the  low-grade  samples  to  find  the  correct  stop- 
ing  value  of  the  ore. 


ESTIMATION     OF     ORE  91 

In  discussing  Garthwaite's  paper,  H.  S.  Munroe  calls  attention*- 
to  the  formula  for  determining  the  probable  error  of  the  arithmet- 
ical mean  of  a  number  of  observations,  and  states :  "The  use  of 
this  formula  for  the  study  of  sampling  data  will  give  valuable  infor- 
mation as  to  the  accuracy  of  the  final  result  and  enable  one  to  deal 
intelligently  with  high  assays.  If  n  =  number  of  observations, 
d  =  difference  of  each  from  the  arithmetical  mean,  then  the 


Probable  error=0.6745 


V  d2! 


n     (n-1) 

In  Garthwaite's  second  example,  Mr.  Munroe  makes  certain  cal- 
culations showing  the  probable  error  by  omitting  one  or  more  of 
the  high  assays,  but  the  deductions  he  is  able  to  make  do  not  clear 
up  the  difficulty. 

No  doubt  the  formula  will  perform  in  a  satisfactory  manner 
the  service  for  which  it  is  intended,  but  applying  it  to  find  the 
probable  error  in  a  number  of  sampling  observations,  will  not 
necessarily  give  a  correct  result.  This  formula  to  be  of  use  ne- 
cessitates as  an  hypothesis  that  the  individual  observations  are 
in  themselves  correct,  although  it  may  seem  to  be  a  paradox,  at- 
tempting to  find  the  probable  error  in  a  number  of  observations, 
each  one  of  which  is  assumed  to  be  correct.  With  the  sampling 
results  with  which  we  are  here  dealing,  there  can  be  no  question  that 
an  error  in  sampling  has  been  made,  as  conclusively  demonstrated 
by  the  entirely  different  results  that  were  obtained  by  the  mine 
management  and  the  independnt  sampling  over  the  same  stretches 
of  ground.  Consequently  any  calculations  based  on  the  result 
of  the  incorrect  sampling  must  likewise  be  incorrect.  This  is  but 
another  illustration  of  what  has  been  pointed  out  so  often  in  these 
pages,  namely,  that  judgment  must  be  used  in  sampling  and  valuation 
work.  No  formula  can  be  a  substitute  for  experience,  and  the  mine 
management  more  familiar  with  the  ore  got  the  more  correct  result. 

Assay  Plans. — When  the  re-sampling  has  been  completed  and 
the  check  assays  noted,  the  preparation  of  an  assay  plan  may  be 
undertaken,  for  not  until  then  are  the  data  for  doing  this  complete. 

When  an  orebody  is  vertical  or  dips  at  a  high  angle,  it  will  be 
found  generally  more  convenient  to  show  the  assays  on  a  longi- 
tudinal projection  or  section  rather  than  on  a  plan,  because  in  the 
plan  of  steeply  inclined  workings,  the  levels  overlap,  or  there  is 
so  little  space  between  them  that  the  necessary  figures  cannot  be 
written  in,  or  at  best  are  so  close  together  as  to  be  confusing. 
Where  the  assays  are  shown  on  a  projection  or  section,  unless  the 
levels  are  practically  straight  and  also  parallel  to  the  plane  of  the 
section,  they  are  shown  foreshortened,  so  that  the  sampling  in- 
tervals cannot  be  platted  to  scale  but  must  be  foreshortened  as 
well.  To  facilitate  this,  they  may  first  be  laid  off  on  a  strip  of 
cross-section  or  other  paper  and  then  transferred  to  the  drawing. 

*Mining  and  Scientific  Press,  Vol.   105;  page  18  (July  6,  1912). 


92  MINE    SAMPLING   AND   VALUING 

•Some  orebodies  are  so  tortuous  that  there  are  curved  lines  not 
only  along  a  level,  but  also  in  the  winzes  and  raises,  on  account  of 
changes  in  dip  at  different  points  along  the  lode.  In  such  case 
not  even  a  plan  in  an  inclined  plane  can  represent  all  the  work- 
ings in  their  true  length,  and  it  may  be  necessary  to  show  assays 
both  on  a  projection  and  in  plan. 

With  orebodies  such  as  the  Rand  banket,  or  other  more  or  less 
regularly  dipping  bodies,  it  is  a  common  practice  to  plat  the  work- 
ings on  the  plane  of  the  lode.  With  a  highly  inclined  orebody  of 
considerable  width  a  separate  plan  of  each  level  may  be  neces- 
sary. The  engineer  then  must  decide  according  to  the  conditions 
which  is  most  suitable,,  namely  to  show  the  assays  on  a  plan  of 
the  workings,  on  a  projection,  or  on  a  longitudinal  section. 

The  assays  once  platted  furnish  a  graphic  representation  of 
the  mode  of  occurrence  of  the  values,  and  if  the  mine  develop- 
ments are  sufficiently  advanced,  show  the  pitch  of  the  ore  chutes 
and  other  phenomena  of  a  like  character  useful  in  a  proper  esti- 
mation of  the  tonnag'e  of  ore  opened  up.  One  level  may  show 
a  persistence  of  values  through  its  entire  length,  and  the  succeed- 
ing one  may  show  a  break,  although  the  lode  matter  continues. 
This  may  be  an  indication  that  the  orebody  is  pinching  out  as 
depth  is  attained.  Likewise  an  intrusion  may  make  its  appear- 
ance on  the  lower  level,  which  does  not  exist  on  the  upper  one. 
The  orebody  may  pinch  in  places  and  the  assay  plan  will  show 
whether  such  phenomena  are  persistent  from  one  level  to  the 
next,  or  are  merely  local.  The  assay  values  taken  in  conjunction 
with  the  geological  features,  which  subject  has  been  discussed 
more  fully  elsewhere,  are  the  chief  factors  in  a  determination  of 
the  potentialities  of  the  mine.  They  must  be  carefully  studied 
before  proceeding  with  the  ore  reserve  calculations. 

There  is  some  divergence  of  method  in  setting  out  the  infor- 
mation on  an  assay  plan  or  section.  My  own  practice  is  to  omit 
the  sample  numbers  and  to  note  the  assays  and  widths  opposite 
the  respective  intervals  they  represent.  The  sample  is  given  a 
number  only  for  the  purpose  of  identifying  it  during  the  various 
stages  from  the  time  it  is  taken  till  it  is  noted  on  the  assay  plan. 
The  number  itself  has  no  excuse  for  existence  on  the  assay  plan, 
except  for  the  possible  contingency  that  one  may  wish  to  refer 
back  to  the  original  underground  notes,  but  even  in  that  ease  the 
sample  can  be  quickly  identified  by  its  position  with  reference  to 
the  nearest  survey  station,  cross-cut,  winze  or  other  convenient 
datum  point. 

The  information  wanted  on  an  assay  map  is  the  assay  value 
and  width  of  the  interval  to  compare  it  with  its  neighbors,  and 
the  picture  conveys  the  full  information  desired.  Where  there 
are  several  metals  the  assays  of  each  can  be  marked  in  a  distinctively 
colored  ink  or  the  same  sequence  from  left  to  right  adopted 
throughout.  I  have  seen  the  assays  and  widths  enclosed  in  a 
circle  and  an  arrow  drawn  to  indicate  the  place  from  which  they 


ESTIMATION     OF     ORE 


93 


have  been  taken.  Some  engineers  even  go  so  far  as  to  make  an 
assay  plan  on  which  the  sample  intervals  are  marked  with  their 
respective  sample  numbers  and  the  assays  and  widths  set  out  in 
a  tabulated  statement.  This  method  detracts  from  the  value  of 
the  information,  as  it  is  difficult  to  follow. 

The  usefulness  of  an  assay  plan  depends  on  the  facility  with 
which  the  information  desired  can  be  obtained.  Because  frac- 
tional samples  have  been  taken  is  no  reason  why  each  individual 
result  should  be  shown  on  the  assay  plan.  Attention  has  already 
been  drawn  to  the  fact  that  fractional  samples  are  taken, 
for  reasons  beyond  the  control  of  the  engineer.  From  the  mining 
viewpoint  what  is  wanted  is  the  average  value  of  the  orebody  at 
the  interval,  and  it  is  immaterial  whether  it  has  been  necessary  to 
take  two  or  twenty  fractional  samples  to  get  the  value  of  the 
interval.  Recently  I  had  occasion  to  investigate  some  assay  plans 
of  a  wide  orebody.  There  were  as  many  as  20  fractions  5  ft. 
wide  to  represent  the  whole  interval,  and  before  an  intelligent 
idea  of  the  values  was  obtained  these  had  to  be  averaged.  If 
there  be  more  than  one  gTade  of  ore,  each  of  which  is  to  be  mined 
separately,  then  the  outlines  of  each  should  be  shown  and  the 
average  of  each  interval  set  down  accordingly. 

Calculation  of  Tonnage. — Preliminary  to  calculating  the  ton- 
nage of  ore  opened  up,  the  mine  is  for  convenience  divided  into 
blocks  or  sections,  whose  size  is  ordinarily  determined  by  the 
physical  features  of  the  mine  workings,  the  ground  bounded  by 
two  levels  and  two  winzes  being  usually  a  determining  factor. 
The  calculations  are  done  in  one  of  the  'Ore  Reserve  Calculations* 
books — and  as  an  accessory  those  for  individual  lengths  may  be 
done  on  strips  of  cross-section  paper,  because  the  ruled  squares 
permit  laying  off  the  intervals  to  scale.  As  the  data  for  a  given 
length  of  ground  may  be  used  twice,  this  permits  bringing  the 
proper  strips  together  as  required.  For  instance,  the  figures  for 
a  level  lying  between  two  blocks,  one  above  and  one  below,  are 
used  for  both  blocks,  the  calculations  for  which  are  made  at  different 
times. 

To  get  the  general  average  value  along  a  length  of  working,  the 
same  general  method  is  employed  as  used  in  averaging  the  re- 
sults of  fractional  samples,  except  that  the  two  terminal  samples 
are  each  given  only  half  the  weight  of  the  others.  The  reason 
for  this  is,  that  a  sample  is  supposed  to  represent  the  ore  half 
way  between  it  and  the  next  sample ;  with  the  terminal  samples 
ore  only  on  one  side  is  taken  into  the  calculation. 

To  take  a  specific  case,  let  us  assume  that  it  is  desired  to  obtain 
the  average  assay  value  of  a  rectangular  block  of  ground,  bounded 
by  two  drifts  100  ft.  apart  and  by  two  winzes  in  the  orebody  200 
ft.  apart. 

From  the  assay  records  the  average  value  of  each  level  and 
each  winze  is  calculated,  care  being*  observed  that  the  terminal 
samples  of  each  lot  are  not  weighted  disproportionately  to  the 


94  MINE    SAMPLING   AND   VALUING 

tonnage  they  represent.  The  method  of  averaging  the  interval 
values  to  ascertain  the  general  average  of  the  section  is  the  same 
as  that  used  for  averaging  the  fraction  assays.  The  assay  value 
of  each  interval  is  multiplied  by  the  width,  giving  a  width  X 
value  factor,  such  as  inch  X  dollar  or  inch-shilling.  The  sum 
of  these  for  all  the  samples  is  divided  by  the  sums  of  the  widths 
of  the  samples  and  the  quotient  is  the  average  assay  of  the  lot. 
The  average  width  is  found  by  dividing  the  sum  of  the  width  by 
the  number  of  samples. 

Where  the  vertical  development  opening's  are  carried  in  the 
orebody  in  the  same  manner  as  the  drives,  then  to  obtain  the  aver- 
age of  the  block  of  ground  the  winzes  and  drives  are  weighted 
relatively  to  their  length.  In  the  case  assumed,  as  the  two  levels 
are  each  assumed  to  be  of  the  same  length  and  also  the  two 
winzes,  the  calculation  may  be  simplified  by  averaging  the  two 
levels  and  the  two  winzes  separately  and  then  taking  the  general 
average  as  shown  here — otherwise  the  method  of  averaging  ne- 
cessitates the  multiplication  of  the  length  X  width  X  assay  for 
each  level  and  each  winze  and  obtaining  the  average  assay  by 
dividing  the  total  of  these  factors  by  the  sum  of  length  X  width 
in  the  same  manner  as  shown  below. 

Take  the  average  of  the  levels  and  winzes  to  be  as  follows : 

Levels     200  ft.  long  average  value  $10  for  5  ft. 
Winzes  100  ft.  high         "          "  8     "    4  " 

Then  200  X  5  X  10  =  10,000  sq  ft.-dollars 
100  X  4  X    8  =    3,200   "   " 


13,200  sq.  ft.-dollars. 

To  get  the  average  value  of  the  block  of  ground,  divide  this 
amount  by  the  sums  of  the  product  of  the  length  of  each  by  the 
width  in  feet,  thus : 

200  X  5  -  1,000 
100  X  4  =     400 

Total  1,400  sq.  ft. 

therefore  13,200  -r-  1,400  =  $9.43  average  value.  To  get  the  aver- 
age width,  divide  the  sum  of  the  lengths  by  the  sum  of  the 
products  of  each  length  by  its  corresponding  width,  thus : 

200  X  5  =  1,000 
100  X  4  -     400 

Totals        300  1,400  sq.  ft. 

Dividing  the  sums  of  the  lengths  into  the  product  we  have : 
1,400  -r-  300  =  4.67  ft.  average  width 

Therefore  this  block  of  ground  200*  ft.  long  by  100  ft.  high 
averages  4.67  ft.  in  width  for  an  average  assay  value  of  $9.43. 

The  cubical  contents  of  the  block  of  ground  is  obtained  by 
multiplying  together  the  three  dimensions,  namely  the  length  by 


ESTIMATION   OF   ORE  95 

the  height  of  the  average  width.  This  gives  the  number  of 
cubic  feet  of  ore  in  the  block,  and  the  number  of  tons  is  found  by 
dividing  the  cubic  feet  by  a  previously  determined  factor,  repre- 
senting the  number  of  cubic  feet  to  the  ton. 

With  quartz  13  to  15  cubic  feet  per  ton  of  ore  in  place  is  or- 
dinarily used,  14  being  a  common  figure.  The  proper  factor  must 
be  determined  in  each  particular  case,  and  if  no  mine  records  are 
available,  then  specific  gravity  determinations  must  be  made  by 
the  engineer  and  due  allowance  made  for  the  cracks,  vugs,  and 
other  open  spaces  in  the  orebody.* 

In  the  case  under  discussion,  let  us  assume  the  ore  to  be  quartz 
and  14  cu.  ft.  of  ore  in  place,  to  weigh  one  ton  of  2,000  Ib. 

Then  200  ft.  long  X  100  ft.  high  X  4.67  ft.  wide  =  93,400  cu.  ft.,  and  93,400  -=- 
14  =  6,671  tons  of  ore,  having  an  average  assay  value  of  $9.43. 

Needless  to  say,  underground  workings  are  not  always  laid 
out  so  symmetrically;  very  often  one  level  in  a  block  is  longer 
than  the  other,  but  the  same  procedure  is  followed,  each  being 
weighted  proportionally  to  the  length  of  face  taken  into  the 
calculation. 

The  volumetric  method  of  averaging  assays,  as  described  above,  is 
the  one  in  general  use  among  engineers.  H.  S.  Munroe,1  in  a  paper 
read  before  the  Mining  and  Metallurgical  Society  of  America,  states 
that  when  it  is  required  to  average  the  assays  of  different  kinds  of  ore 
varying  considerably  in  specific  gravity  this  method  may  give  incor- 
rect results,  as  under  such  circumstances  it  may  be  necessary  to  take 
the  specific  gravity  into  the  calculations  as  a  factor.  The  publication 
of  this  paper  serves  to  bring  forward  a  most  important  theory  and 
one  of  which  engineers  in  general  have  as  yet  made  but  little  use. 
Since  the  date  of  its  publication,  I  have  had  the  opportunity  of  discuss- 
ing this  question  with  its  author  at  considerable  length  by  exchange  of 
letters,  and  in  this  correspondence  Mr.  Munroe  amplifies  the  theory 
and  gives  the  method  a  much  wider  application  than  is  to  be  inferred 
from  the  original  article.  In  fact,  he  now  proposes  to  apply  the 
gravimetric  method  in  the  calculation  of  averages,  wherever  there  .is 
any  difference  in  the  specific  gravity  of  the  constituent  minerals  in 
the  ore. 

The  theory  underlying  his  method  is  contained  in  the  following 
quotation  from  one  of  his  letters  :2 

"If  we  have  a  vein  with  ore  of  uniform  density,  varying  in  width 
from  2.6  to  4.0  ft.,  there  is  no  question  that  we  should  weight  the 
samples  according  to  the  width  of  the  vein,  as  the  wide  places  repre- 
sent more  tonnage  than  the  narrow.  If  we  have  a  vein  uniform  in 
width  but  varying  from  2.6  to  4.0  in  specific  gravity,  it  is  equally  true 
that  the  samples  of  high  specific  gravity  represent  a  higher  tonnage  of 
ore." 


*See  chapter  on  specific  gravity  determinations   (page   137). 
^Mining  and  Scientific  Press,    Vol.    105;   p.    18    (July   6,    1912). 
'Letter  dated  April  29,    1913. 


96 


MINE    SAMPLING   AND   VALUING 


On  the  face  of  it,  this  is  a  most  convincing  statement  and  would 
seem  to  leave  but  little  room  for  argument. 

Before  proceeding  with  the  discussion  of  this  theory,  it  will  be 
useful  to  quote  one  of  the  cases  cited  by  Mr.  Munroe  in  his  paper,  to 
show  the  application  of  the  method.  He  states : 

"For  the  purpose  of  illustration,  I  assume  five  samples  taken  at 
uniform  distances  in  a  galena  vein,  varying  in  thickness  from  10  to 
60  in.,  and  the  ore  varying  from  10  to  80%  of  lead. 


Thickness, 
Inches. 

60 
20 
50 
10 
10 


Lead, 
Per  cent. 

10 

40 
25 
80 
60 


Inch. 
Per  cent. 

600 
800 
1250 
800 
600 


5/150 
Averages  30 


5/215 

43.0 
(Arithmetic) 


150/4050 

27.0 
(Volumetric) 


"Calculating  the  average  in  the  usual  way  by  multiplying  the 
thickness  in  inches  by  the  percentage  of  lead  and  dividing  by  the 
total  number  of  inches,  an  average  value  of  27%  is  found,  which 
may  be  called  the  volumetric  average.  This  is  less  than  the  arith- 
metic average,  which  in  this  case  is  43%.  Neither  of  these  results, 
however,  is  correct,  as  the  result  obtained  by  sampling  must  be 
given  weight  in  proportion  to  the  tons  of  ore  which  each  sample 
represents.  In  the  following  table  I  have  introduced  specific  gravity 
as  well  as  thickness : 


Thickness, 

Specific 

Inch- 

Lead,                 Inch- 
Gravity 

Inches 

Gravity.          Gravity. 

Per  cent.         Per  cent. 

60 

X 

3.0        = 

ISO 

X 

10        = 

1800 

20 

X 

5.0        = 

100 

X 

40         = 

4000 

50 

X 

4.0        = 

200 

X 

25         = 

5000 

10 

X 

7.5        = 

75 

X 

80        = 

6000 

10 

X 

6.0        = 

60 

X 

60        = 

3600 

150 


150/615 


615/20400 


Average  spec.  grav.  4.1          Average  per  cent  33.17 


"If  the  thickness  in  inches  is  multiplied  by  the  specific  gravity 
of  the  sample,  a  product  is  obtained  which  may  be  called  the  inch- 
gravity  value,  which  is  a  unit  proportional  to  the  tonnage  repre- 
sented by  the  sample.  Multiplying  this  by  the  percentage  of  lead 
found  by  assay  gives  an  inch-gravity-per-cent  product.  Dividing 
the  sum  of  these  last  products  by  the  total  of  the  inch-gravity 
column  gives  as  the  average  33.17%,  which  lies  between  the  volu- 
metric and  the  arithmetic  average.  This  may  be  called  the 
gravimetric  average." 


ESTIMATION   OF   ORE  97 

In  the  averaging  of  assays,  we  are  really  confronted  with  two 
problems,  first  the  averaging  of  fractional  samples  and  the  other 
the  averaging  of  interval  samples.  The  first  problem  is  really  doing 
by  mathematics  what  we  actually  accomplish  in  the  operation  of 
cutting  the  sample,  that  is,  we  endeavor  to  get  such  quantities  from 
each  unit  of  width  of  the  ore  as  will  be  proportional  to  the  actual 
tonnages  that  will  be  mined  later  on.  In  cutting  the  sample,  as  has 
already  been  explained  in  a  previous  chapter,  these  quantities  are 
taken  by  volume ;  therefore,  those  constituent  minerals  which  have 
the  highest  specific  gravity  will  comply  with  the  condition  laid  down 
in  Mr.  Munroe's  theory  and  influence  the  result  to  the  greatest  ex- 
tent; in  other  words,  an  equal  volume  of  heavy  mineral  will  add 
a  greater  weig'ht  to  the  sample  than  a  light  mineral. 

What  is  meant  to  be  conveyed  by  the  above  is  that  in  any  ore- 
body  where  there  is  a  mixture  of  minerals,  if  the  sampling  is 
properly  done,  representative  amounts  must  be  taken  and  only  in 
case  the  ore  is  banded  in  such  a  manner  that  separate  samples  may 
be  taken  of  the  different  bands,  can  the  specific  gravity  factor 
seriously  influence  the  result. 

In  averaging  interval  samples  a  somewhat  different  condition 
exists.  Mr.  Munroe  shows,  although  citing  an  extreme  case,  how 
much  the  element  of  gravity  may  affect  the  result.  On  the  other 
hand,  in  his  correspondence  he  advances  arguments  in  favor  of  its 
use  in  every-day  work  with  any  kind  of  ore  where  there  is  any 
mixture  of  minerals.  He  states  :* 

"My  plan  of  operations  in  the  case  of  a  concentrating  ore,  for 
example,  in  which  the  valuable  mineral  is  scattered  through  the 
vein  rilling,  is  to  cut  a  groove  with  the  moil  across  the  vein  from 
foot  to  hanging  wall.  This  we  will  consider  one  sample.  Five  or 
ten  feet  away,  as  the  case  may  be,  I  cut  another  sample  in  a  similar 
manner,  and  so  on.  In  each  sample,  I  determine  the  average  specific 
gravity  of  mineral  and  gangue  together  (not  separately),  using  for 
this  purpose  an  average  sample  from  one  of  the  rejected  portions  of 
the  moil  sample  when  working  the  whole  down  to  bottle  sample 
for  analysis.  The  sample  upon  which  the  specific  gravity  deter- 
mination is  made  thus  represents  the  average  specific  gravity  of  the 
moil  sample  from  one  wall  to  the  other.  The  same  thing  will  be 
done  with  each  moil  sample,  so  that  we  shall  have  as  many  deter- 
minations of  specific  gravity  as  samples  are  taken  in  the  mine,  sev- 
eral hundreds  or  several  thousands  perhaps.  In  averaging  the  assay 
values  I  do  not  depend  on  the  measurement  of  the  vein  from  foot  to 
hanging  wall  alone,  but  multiply  this  measurement  in  each  case  by  the 
specific  gravity  of  the  corresponding  sample,  obtaining  thereby  an 
inch-gravity  factor  which  is  proportional  to  the  tonnage  and  not  to  the 
volume  which  each  sample  represents.  In  other  words,  my  final 
average  is  a  gravimetric  one  and  not  a  volumetric  one,  as  is  usually 
the  case. 

*Letter    dated    May    16,    1913. 


98  MINE    SAMPLING   AND   VALUING 

"It  is  not  true  that  the  difference  between  the  volumetric  and 
gravimetric  methods  will  be  negligible  even  with  a  small  percentage 
of  valuable  mineral  present.  The  common  gangue  minerals, 
silicates,  carbonates,  etc.,  of  the  vein  filling  differ  very  greatly  in 
specific  gravity.  You  have  only  to  refer  to  Dana  to  establish  this 
fact.  Different  samples  of  ore  from  the  same  level  will  vary  in 
gravity  with  the  proportion  of  one  or  another  gangue  mineral 
present.  Again  the  vein  filling  often  consists  largely  (sometimes 
wholly)  of  barren  sulphides  or  sulphides  containing  small  percent- 
age of  valuable  mineral  only.  The  presence  of  a  greater  or  less 
quantity  of  these  low-grade  sulphides  (which  in  such  case  are  as  much 
gangue  minerals  as  the  silicates  which  may  be  present  at  the 
same  times),  does  affect  very  materially  the  specific  gravity  of  the  ore; 
and  adjoining  moil  samples  with  equal  vein  width  represent  great 
differences  in  tonnage — as  great  as  those  due  to  width  which  we  are 
accustomed  to  take  into  consideration.  For  example,  the  con- 
centration ratio  at  Great  Falls  and  Anaconda  is  about  3  to  1.  The 
concentrate  is  almost  wholly  mixed  with  metallic  sulphides  and  the 
tailings  silicious  material.  The  concentrate  does  not  contain  more 
than  10%  of  copper,  so  that  the  metallic  sulphides  in  the  ore  and 
in  the  concentrate  are  largely  gangue.  Any  concentrating  ore 
which  contains  so  large  a  percentage  of  metallic  sulphides,  must 
vary  greatly  in  specific  gravity  all  through  the  mine,  the  variation 
perhaps  running  from  that  of  the  silicates  with  little  or  no  sul- 
phides to  clean  sulphides  with  little  or  no  silicate.  The  percent- 
age of  the  valuable  mineral,  in  this  case  copper,  will  vary  but 
little,  say  one  per  cent  to  five  per  cent.  Such  a  mine  would  give 
as  extreme  a  case  as  any  that  I  have  indicated  in  my  original  paper. 
But  we  do  not  have  to  consider  such  extreme  cases.  I  venture  to 
say  that  the  ore  of  every  mine,  whether  containing  sulphides  or 
not,  will  be  found  to  show  wide  variations  in  specific  gravity,  if 
specific  'gravity  determinations  be  made  on  every  sample,  as  I 
propose.  If  I  am  right  in  this,  my  method  will  have  universal 
application  wherever  we  desire  to  introduce  all  possible  pre- 
cautions to  secure  the  most  accurate  results  practicable.  This  is 
my  attitude  at  all  times,  especially  when  it  adds  little  to  the  work 
and  little  to  the  expense,  as  in  this  case." 

That  the  use  of  this  method  will  require  an  additional  amount 
of  expense  and  time  is  no  valid  reason  against  its  application,  pro- 
vided it  gives  a  more  accurate  result  than  any  other.  Taking  the 
case  of  the  galena  orebody  just  quoted  as  an  illustration,  it  will 
be  seen  that  the  gravimetric  average  gives  a  considerably  higher 
result  than  the  volumetric,  and  in  any  similar  cases,  similar  results 
must  be  obtained,  of  course,  varying  only  in  degree.  No  doubt 
conditions  might  be  assumed  where  the  gravimetric  average  would 
show  a  lower  result  than  the  volumetric,  but  certainly  by  far  the 
largest  number  of  cases  would  show  a  higher  result. 

It  is  a  well  known  fact,  which  has  been  fully  explained  in  the 
preceding  pages,  that  the  assay  of  ore  as  mined  is  usually  lower 


ESTIMATION   OF   ORE  99 

than  shown  by  the  sampling  of  the  ore  before  it  is  broken.  It 
will  be  recalled  that  in  the  case  of  the  Rand  this  difference  amounts 
to  as  much  as  25%.  If  engineers  were  in  a  position  to  make  a 
proper  allowance,  it  is  safe  to  say  that  it  would  be  done.  Every 
engineer  in  assessing  the  assay  value  of  an  ore  deposit,  always 
makes  an  allowance  for  the  admixture  of  country  rock  with  the 
broken  ore  and  yet  this  difference  continues  to  exist,  so  much  so 
that  most  engineers  will  state  that  an  orebody  cannot  be  mined 
as  clean  as  it  is  customary  to  sample  it.  Of  course,  the  object  of 
mine  sampling  must  be  to  determine  the  stoping  value  of  the 
orebody,  and  proper  scientific  methods  aim  at  making1  these 
sampling  results  correspond  with  the  stoping  results. 

The  error  being  all  on  one  side,  leads  one  to  the  conclusion 
that  the  difference  is  largely  beyond  the  control  of  the  engineer 
and  that  despite  any  precautions  that  may  be  taken,  an  excess  of 
the  brittle  mineral,  which  is  usually  the  richest,  enters  the  sample. 
This  brittle  mineral  is,  of  course,  usually  a  sulphide  with  a  higher 
specific  gravity  than  the  ordinary  silicious  or  other  non-metallic 
gangue  and  this  may  account  for  the  sampling  results  being  higher 
than  the  stoping1  results. 

The  application  of  the  gravimetric  method  in  the  majority  of 
the  cases  will  increase  this  condition  and  necessitate  the  applica- 
tion of  a  larger  factor  of  reduction. 

There  is  no  use  in  altering  one's  methods  unless  they  show  an 
improvement  over  the  ones  that  have  previously  been  in  use.  While 
I  am  frank  to  admit  that  the  argument  in  favor  of  the  gravimetric 
method  is  an  extremely  strong  one,  yet  considering  the  greater 
amount  of  work  involved  when  using  it  and  the  fact  that  appar- 
ently it  does  not  bring  us  nearer  to  the  stoping  values,  makes  me 
hesitate  in  recommending  its  use.  On  the  other  hand,  I  feel  that 
the  method  has  not  been  sufficiently  tested  to  demonstrate  its 
usefulness,  or  otherwise,  and  until  greater  experience  is  had  with 
it,  judgment  must  be  suspended. 

Inaccessible  Ore. — We  now  come  to  the  discussion  of  certain 
problems,  arising  from  the  interpretation  of  the  assay  results.  It 
may  eventuate  that  portions  of  the  workings  have  not  been  sampled 
because  inaccessible  on  account  of  caved  ground,  timber,  or  other 
cause.  The  question  arises  as  to  how  these  shall  be  taken  into  the 
ore  reserve  calculations.  It  may  be  arbitrarily  stated  that  where 
no  samples  are  taken,  no  value  should  be  allowed,  but  this  is  often 
manifestly  unjust.  Of  course,  if  good  grade  ore  is  reported  to 
exist  behind  timber,  while  on  either  side  low-grade  ore  or  barren 
ground  is  exposed,  such  statement  must  be  looked  upon  with 
suspicion  and  the  weight  of  evidence  would  lead  the  engineer  to 
omit  this  stretch  of  ground  from  his  tonnage  calculations. 

On  the  other  hand,  where  there  is  a  stretch  of  timbered  ground, 
on  both  sides  of  which  is  ore  of  minable  grade,  the  engineer  may 
perhaps  be  justified  in  assigning  to  this  portion  the  average  value 
of  the  neighboring  ore,  provided  that  it  is  not  too  long.  It  must 


100  MINE    SAMPLING   AND    VALUING 

be  borne  in  mind  that  the  working  may  have  been  timbered  pur- 
posely to  obscure  an  unprofitable  stretch  of  ground.  Here  again 
judgment  is  called  for,  and  if  there  is  any  doubt,  the  tonnage  corre- 
sponding to  this  length  should  be  reduced  by  a  definite  percentage 
according  to  circumstances  or  preferably  omitted  altogether.  It 
will  be  noted  that  a  reduction  in  the  tonnage  is  suggested  and  not 
the  value,  although  that  may  be  deemed  advisable  as  well. 

At  times  the  engineer  has  the  result  of  a  previous  examination 
or  the  mine  records  to  guide  him.  As  an  illustration  of  the  diffi- 
culty that  besets  one  and  the  risk  in  making1  assumptions  of  this 
character,  I  recall  a  case  where  the  result  of  a  previous  examina- 
tion showed  lower  assay  values  in  ground  that  was  timbered  at 
the  time  of  the  second  engineer's  visit  than  the  average  of  the 
neighboring  ore.  In  such  case  the  lower  values  of  the  former 
examination  may  be  used,  providing  the  work  was  done  by  a 
reliable  engineer. 

If  the  procedure  suggested  above  is  followed  and  the  margin 
of  profit  in  the  ore  is  not  too  small,  any  discrepancy  arising  there- 
from is  not  likely  seriously  to  affect  the  profit  calculated  for  the 
block,  because  the  deduction  made  as  a  factor  of  safety  simply  gives 
a  smaller  tonnage  of  higher  grade  ore.  If  a  sufficiently  conserva- 
tive deduction  has  been  made  the  net  profit  shown  is  not  likely 
to  be  very  different  except  in  the  single  instance  where  the  margin 
of  working  profit  per  ton  of  ore  is  very  small.  In  such  an  event 
this  feature  must  be  given  due  weight  in  the  engineer's  calculations 
and  he  must  not  hesitate  if  circumstances  warrant  in  omitting  such 
a  section  altogether. 

It  is  in  cases  of  this  sort  that  the  skill  of  the  engineer  is  brought 
into  play.  He  must  be  guided  by  the  geological  evidence  as  well 
as  the  mode  of  occurrence  of  values,  to  enable  him  to  form  a  proper 
opinion  and  in  the  absence  of  positive  evidence  as  to  values  he 
must  err,  if  at  all,  on  the  side  of  safety.  Better  be  sure  than  sorry ! 

Irregularly-Spaced  Drill  Holes. — In  churn-drilling  the  dissem- 
inated copper  deposits  which  form  the  basis  of  the  operations  described 
in  Chapter  IX,  expense  is  not  spared  in  preparing  the  ground,  build- 
ing roads,  etc.,  in  order  to  space  the  holes  at  regular  intervals.  Some- 
times drilling  tabular  deposits  cannot  be  carried  out  in  such  a  sys- 
tematic manner,  owing  to  physical  conditions,  or  other  causes,  in 
which  case  the  method  of  averaging  up  the  assays  must  be  modified 
to  meet  such  irregularity. 

To  average  the  assays  of  holes  spaced  at  regular  intervals,  the 
volumetric  average  of  the  samples  is  taken  in  the  same  manner  as 
with  samples  at  regular  intervals  in  ordinary  underground  workings ; 
that  is,  the  interval  or  distance  between  the  samples  is  not  taken  in  as 
a  mathematical  factor,  because  it  influences  each  sample  in  a  like  pro- 
portion. On  the  other  hand,  with  irregularly  spaced  drill  holes,  the 
distance  between  the  holes  must  be  allowed  for,  as  otherwise  import- 
ant differences  may  result. 


ESTIMATION   OF   ORE 


101 


The  theory  underlying  the  sampling  interval  and  its  influence  on 
the  general  result  has  already  been  discussed  elsewhere.  Each  sam- 
ple is  supposed  to  represent  the  block  of  ground  half  way  to  the  next 
sample,  and  the  same  rule  must  apply  in  considering  the  results 
from  drilling  operations.  In  the  case*  of  irregularly  spaced  holes,  a 
trapezoid  must  be  .considered,  whose  dimensions  are  the  mean  thick- 
ness of  ore  at  the  two  holes  and  the  distance  between  them.  The 
samples,  of  course,  must  be  weighted  in  the  calculations  in  direct  pro- 
portion to  the  quantity  of  ore  they  represent;  hence  the  area  of  the 
trapezoid  multiplied  by  the  volumetric  average  assay  of  the  two  holes 
gives  what  may  be  called  the  area  X  per  cent  factor. 

The  following  example  will  indicate  how  easily  miscalculations 
may  occur  if  the  results  of  the  drilling  are  not  properly  interpreted, 
and  the  assays  weigtited  proportionally  to  the  averages  they 
represent. 

Assume  four  holes  put  down  in  a  copper  deposit  by  churn  drill, 
or  diamond  drill,  and  spaced  as  per  sketch. 


Qc-- 

A 


3/5-  ff. 


B 


Assay //Z'/o 


Depth /If I- 
Assay  d  9% 


Depth  8  ft 
Assay  /7.Z  'A 


Depfh  Gfr 
Assay 5.5% 


Fig.   21. 


Thus  we  have  average  depth  and  assay  of  adjoining  holes. 


Foot 
Depth,  Assay            X 
Ft.    per  cent,    per  cent. 

4X11.2=          44.8 
11  X    8.9=         97.9 

Foot 
Depth,  Assay            X 
Ft.    per  cent,    per  cent. 

11  X    8.9=          97.9 
8  X  17.2  =        137.6 

Foot 
Depth,  Assay            X 
Ft.    per  cent,    per  cent. 

8  X  17.2  =        137.6 
6  X    5.5=         33.0 

2/15                  15/142.7 

19                  19/235.5 

14                  14/170.6 

7.5  ft.  av.  dpth.  Av.  9.5% 

9.5  ft.  av.        Av.  12.4% 

7  ft.  av.        Av.  12.2% 

To  get  the  average  of  the  four  holes,  allowing  only  for  the  ground 
within  them,  there  follows : 


Aver. 

Depth 

Ft. 


Distance 

Between 

Holes. 


7.5  X  240 
9.5  X  315 
7.0  X  160 

24.  715 

8.3  ft.  average  depth. 


Area  of 

Trape-  Average 

2  oid.  Assay. 

1800.0  X          9.5 

2992.5  X         12.4 

1120.0  X         12.2 


Area 

X 
Per  cent. 

17100 
27107 
13664 


/5912.5 


5912.5 


Real 

/57871/9.8%      Average 


102  ^  MtNE    SAMPLING   AND   VALUING 

As  compared  to  this  result,  by  omitting  the  distance  between  the 
holes,  we  have  the  following: 

Assay 

Depth  at  ' 

Hole.  Feet.  -Hole. 

A.  4  X  11.2  44.8 

B.  11  X  8.9  97.9 

C.  8  X  17.2  137.6 

D.  6  X  5.5  =  33.0 

4/29  29/313.3/10.8%  average. 

"7.3 

This  shows  a  difference  of   1   unit  of  copper,  or   10%,   approxi- 
mately, of  the  total. 

If  we  only  weight  the  two  end  holes  one-half  we  get : 

A.  2  v  n.2  22.4 

B.  11  8.9  97.9 

C.  8  X  17.2  137.6 

D.  3  X  5.5  16.5 


24  24/274.4 

11.4%  average. 
making  a  difference  of  1.6  units  of  copper. 

Vary  the  assays  and  the  dimensions,  and  different  results  follow. 
Whether  the  two  end  holes  shall  be  given  only  half  weight  must  be 
decided  in  each  individual  case,  depending  on  geological  conditions. 
The  differences  do  not  always  work  out  in  the  one  direction,  as  the 
real  average  may  be  more  than  the  incorrect  one. 

The  volume  of  ore  included  within  certain  drill  holes  is  determined 
by  means  of  the  prismoidal  formula,  commonly  used  for  calculating 
cuts  and  fills  in  railroad  work.  The  volume  being  known,  the  ton- 
nage follows  by  dividing  the  amount  by  the  factor  representing  the 
number  of  cubic  feet  per  ton  of  ore  in  place. 


CHAPTER  XII. 

ORE  IN  SIGHT. 

Definitions.— If  the  estimation  of  ore  merely  consisted  of  cal- 
culating the  contents  and  assay  value  of  ground  opened  on  four 
sides,  the  operation  would  be  comparatively  simple.  The  deter- 
mination of  what  properly  constitutes  'Ore  in  Sight'  requires  the 
utmost  skill  and  experience  on  the  part  of  the  engineer,  because  a 
knowledge  of  the  subsequent  metallurgical  treatment,  as  well  as 
the  economic  conditions  governing  the  operations,  is  essential. 
The  monetary  value  of  the  'Ore  in  Sight'  governs  the  worth  of  the 
mine,  hence  its  proper  determination  is  a  most  important  factor 
in  mine  valuation. 

The  problems  that  confront  us  are — how  far  beyond  the  existing 
mine  openings  can  ore  be  assumed  to  continue,  and  can  it  be  as- 
sumed to  be  continuous  between  the  openings? 

What  is  'Ore  in  Sight'? 

In  1902,  as  a  sequel  to  the  discussion  of  a  paper  presented  by 
J.  D.  Kendall,  the  Institution  of  Mining  and  Metallurgy  issued  a 
circular  to  its  members,  recommending  the  following  use  of  the 
term  'Ore  in  Sight': 

1.  That  members  of  the  Institution  should  not  make  use  of 
the  term  'Ore  in  Sight,'  in  their  reports,  without  indicating,  in  the 
most  explicit  manner,  the  data  upon  which  the  estimate  is  based ; 
and  that  it  is  most  desirable  that  estimates  should  be  illustrated 
by  drawings. 

2.  That  as  the  term  'Ore  in  Sight'  is  frequently  used  to  in- 
dicate two  separate  factors  in  an  estimate,  namely : 

(a)  Ore  blocked  out,  that  is,  ore  exposed  on  at  least  three  sides 
within  reasonable  distance  of  each  other, 

(b)  Ore  which  may  be  reasonably  assumed  to  exist  though  not 
actually  blocked  out, 

these  two  factors  should  in  all  cases  be  kept  distinct,  as  (a)  is 
g'overned  by  fixed  rules,  while  (b)  is  dependent  upon  individual 
judgment  and  local  experience. 

3.  That  in  making  use  of  the  term  'Ore  in  Sight'  an  engineer 
should  demonstrate  that  the  ore  so  denominated  is  capable  of  being 
profitably  extracted  under  the  working  conditions  obtaining  in  the 
district. 

4.  That  the  members  of  the  Institution  be  urged  to  protect  the 
best  interests  of  the  profession  by  using  their  influence  in  every 
way  possible  to  prevent  and  discourage  the  use  of  the  term  'Ore 
in  Sight'  except  as  defined  above ;  and  the  Council  also  strongly 
advise  that  no  ambiguity  or  mystery  in  this  connection  should  be 


104  MINE    SAMPLING   AND   VALUING 

tolerated,  as  they  (the  Council)  consider  that  such  ambiguity  is 
an  indication  of  dishonesty  or  incompetence. 

This  was  a  step  in  the  right  direction  and  has  served  a  most 
useful  purpose,  not  only  by  making  engineers  more  careful,  but 
has  been  of  educational  value  to  the  general  public. 

Other  terms  are  used,  but  probably  those  most  commonly  in  use 
must  be  credited  to  B.  B.  Lawrence,  and  form  a  contribution  to 
T.  A.  Rickard's  'Sampling  and  Estimation  of  Ore  in  a  Mine.'  They 
are  as  follows : 

1.  Positive  ore — Ore  exposed  on  all  sides. 

2.  Probable  ore — Ore  exposed  on  two  or  three  sides. 

3.  Possible  ore — Ore  below  the  lowest  level,  or,  as  Mr.  Argall 
expresses  it,  "below  the  last  visible  ore." 

To  my  mind,  neither  of  these  sets  of  definitions  is  satisfactory. 
From  a  technical  standpoint,  the  rules  laid  down  by  the  Institution 
leave  a  great  deal  to  be  desired,  and  the  reasons  given  why  the  two 
factors  (a)  and  (b)  should  be  kept  distinct  appear  to  me  to  be 
incorrect.  The  whole  question  of  the  estimation  of  'Ore  in  Sight' 
must  always  be  dependent  most  largely  on  the  personal  equation 
of  the  engineer  making  the  estimate.  We  cannot  apply  fixed  rules 
to  serve  as  a  guide ;  otherwise  a  novice  could  do  the  work,  instead 
of  it  requiring  great  skill  and  experience.  Local  experience,  as 
suggested  by  the  Institution,  is  helpful,  but  in  most  cases  the  mining 
engineer  has  no  local  experience,  and  must  depend  on  his  general 
experience  to  guide  him,  notwithstanding  which  he  is  required  to 
arrive  at  a  definite  result.  Mr.  Lawrence's  definitions  are  altogether 
too  restrictive  and  do  not  allow  sufficiently  for  varying  conditions. 
It  is  an  attempt  to  arbitrarily  define  that  which  must  depend  on 
individual  judgment. 

Ore  exposed  on  three  or  four  sides  cannot  always  be  reckoned 
as  positive  ore  or  proved  ore,  for  we  can  easily  imagine  the  mine 
openings  at  such  distances  from  each  other  that  no  conservative 
engineer  would  attempt  to  classify  the  included  orebody  as  such. 
Geological  conditions  and  character  of  mineralization  must  be 
taken  into  account.  In  the  case  of  an  iron  mine,  or  a  low-grade 
disseminated  copper  deposit,  we  may  be  willing  to  accept  the  re- 
sults of  holes  drilled  at  intervals  of  200  ft.  as  sufficient  evidence  to 
designate  the  ore  as  proved  ore,  and,  on  the  other  hand,  we  may 
be  unwilling  to  so  classify  a  body  of  gold-bearing  quartz,  of  high 
grade,  that  was  opened  up  with  levels  150  ft.  apart  and  intersected 
by  winzes  at  intervals  of  200  or  300  feet.  In  the  impregnation  type 
of  deposit,  such  as  those  at  Goldfield  and  Tonopah,  Nevada,  or  of 
the  Talisman  mine  at  Karangahake  in  New  Zealand,  it  certainly 
would  be  risky  to  follow  this  procedure  with  any  such  distance 
between  openings. 

The  definitions  laid  down  by  H.  C.  Hoover  in  his  'Principles  of 
Mining'  seem  to  me  to  be  much  more  satisfactory  than  those  of  the 
Institution  of  Mining  and  Metallurgy,  because  they  are  not  re- 
strictive and  fully  recognize  that  the  assessment  of  ore  tonnage 


ORE  IN  SIGHT  105 

is  a  matter  of  personal  equation  and  cannot  be  bound  by  any  hard 
and  fast  rules.  Mr.  Hoover's  classification  is  as  follows : 

Proved  ore:  Ore  where  there  is  practically  no  risk  of  failure  of 
continuity. 

Probable  ore:  Ore  where  there  is  some  risk,  yet  warrantable  jus- 
tification for  assumption  of  continuity. 

Prospective  ore:  Ore  which  cannot  be  included  in  the  above  classes, 
nor  definitely  known  or  stated  in  any  terms  of  tonnage. 

Proved  ore  instead  of  positive  ore  is  more  expressive ;  besides, 
the  only  positive  ore  in  a  mine  is  that  which  is  already  mined. 

The  expression  'probable  ore'  is  one  whose  use  has  been  sanc- 
tioned by  long  practice,  but  in  view  of  the  fact  that  the  general 
custom  in  valuing  mines,  is  to  combine  the  tonnages  and  available 
.  profit  of  both  proved  and  probable  ore,  to  arrive  at  the  total  profit 
available  and  as  a  measure  of  the  price  which  may  be  paid  for  the 
mine,  it  seems  to  me  that  in  its  present  form  of  application  it  has 
a  poor  excuse  for  existence.  If  we  treat  probable  ore  like  proved 
ore,  then  from  the  valuation  standpoint  they  do  not  materially  differ 
one  from  the  other. 

The  expression  'possible  ore'  in  itself  seems  to  me  to  be  par- 
ticularly objectionable,  because  most  people  know  that  in  mining 
all  things  are  possible,  and  that  the  tendency  is  all  on  the  side  of 
the  falsification  of  indications.  On  the  other  hand,  'prospective' 
ore  is  to  be  commended,  because  it  again  implies  the  necessity  of 
judgment  on  the  part  of  the  individual  and  is  a  better  mining  ex- 
pression. 

No  definite  rules  can  be  laid  down  to  guide  the  inexperienced  as 
to  how  far  from  the  sample  face  toward  the  centre  of  a  block  of 
ground  is  permissible  for  ore  to  be  called  proved  ore.  It  depends 
entirely  on  the  character  of  the  deposit.  The  two  extreme  cases 
already  quoted  will  give  some  idea  of  the  difficulty  involved.  Each 
man  must  be  guided  by  his  own  experience.  A  not  uncommon  prac- 
tice with  engineers  in  ordinary  base-metal  deposits  carrying  precious 
metals,  or  medium-grade  gold  deposits,  is  to  consider  as  proved, 
ore  which  is  included  between  levels  100  ft.  to  125  ft.  apart. 

The  number  of  winzes  necessary  is  largely  a  matter  of  geological 
conditions.  It  will  readily  be  appreciated  that  some  winzes  are 
necessary,  in  order  to  prove  that  the  ore  does  not  occur  in  floors, 
or  in  no  other  way  except  as  a  continuous  body.  With  a  mine  having 
levels  100  ft.  to  125  ft.  apart  and  winzes  200  ft.  to  300  ft.  apart, 
I  think  the  usual  practice  will  approve  of  calling  all  the  ore  in- 
cluded within  such  openings  as  'proved'  ore.  (In  special  cases 
even  these  distances  might  be  considered  excessive.) 

Probable  ore  is  a  guess  on  which  a  definite  value  is  placed. 
When  an  engineer  includes  a  definite  tonnage  of  probable  ore  in 
his  calculations,  he  is  simply  backing  his  opinion,  that  the  values 
will  continue  sufficiently  beyond  the  limits  of  the  existing  work- 
ings, to  demonstrate  that  tonnage.  It  is  customary  with  many 
engineers,  where  the  headings  are  still  in  ore,  to  allow  a  certain 


106  MINE   SAMPLING   AND   VALUING 

distance  beyond  the  faces  for  a  further  extension  of  the  orebody. 
Included  in  'probable'  ore,  too,  is  the  extension  of  the  orebody  in 
depth,  which  also  is  a  guess,  based  on  empiricism. 

More  risk  is  involved  in  assuming  the  extension  of  the  ore 
along  the  strike,  beyond  the  faces  of  the  drives,  than  in  assuming 
its  extension  in  depth.  Experience  teaches  us  that  there  are 
orebodies  of  all  lengths  and  widths,  yet  there  is  no  relation  between 
the  length  and  width  of  an  orebody,  that  will  guide  us  in  making 
an  estimate  of  this  kind.  On  the  other  hand,  as  a  result  of  the 
same  experience,  we  have  come  to  believe  that  usually  in  fissure 
veins  there  is  a  more  or  less  definite  relation  between  the  length 
of  the  ore-shoot  and  its  extension  in  depth.  This  put  in  figures 
would  indicate  a  ratio  of  about  1  to  1  or  iy3,  that  is,  if  an  ore- 
body  in  a  fissure  vein  is  1,000  ft.  long,  it  may  be  expected  to  ex- 
tend to  a  depth  of  1,000  ft.  or  1,300  ft.  With  types  of  deposit' 
other  than  fissure  veins,  we  have  no  guide  except  the  experience 
of  the  engineer  and  his  ability  to  size  up  the  geological  condi- 
tions. The  cases  of  the  Bellevue  and  Vivien  mines  in  Western 
Australia  have  already  been  quoted. 

In  a  formation  like  the  Rand  banket,  one  would  be  justified 
in  assuming  as  probable  ore  an  amount  which  would  not  be 
advisable  in  any  other  type  of  deposit,  for  here  we  have  extensive 
developments,  not  only  along  the  strike  of  the  deposit  but  also 
workings  in  depth  have  proved  a  regularity  in  value  which  serves 
as  a  guide  to  the  engineer  in  his  estimates.  At  the  great  native 
copper  mines  of  Lake  Superior,  despite  the  fact  that  the  Calumet 
and  Hecla  lode  has  been  worked  for  a  number  of  miles  on  its 
strike  and  for  a  depth  of  more  than  two  miles  on  its  dip,  with 
value  throughout,  the  deep  shafts  of  the  Tamarack  company  sunk 
to  depths  of  over  5,000  ft.  vertically,  in  order  to  work  the  deeper 
portions  of  this  deposit,  have  been  unprofitable.  The  grade  of  the 
ore  at  that  depth  had  decreased.  These  shafts  were  sunk  for  what 
the  capable  engineers  in  charge  of  the  operations  considered  was 
probable  ore,  and  enormous  sums  of  money  were  expended  in  the 
attempt  to  realize  on  this  probable  ore,  which  the  development 
proved  to  be  unprofitable. 

The  problem  with  which  the  engineer  has  ordinarily  to  contend 
is  simpler  than  this.  From  a  study  of  the  geological  conditions 
and  a  consideration  of  the  amount  of  ground  that  has  been  opened 
up,  he  must  estimate  how  far  the  ore  in  the  mine  will  extend  be- 
yond the  existing  openings.  This  will  represent  what  he  con- 
siders probable  ore.  Whether  he  will  take  into  his  calculations 
the  entire  tonnage  estimated  in  this  manner  depends  on  the  ele- 
ment of  risk  involved.  A  man  may  feel  morally  certain  that  the  ore 
exposed  on  the  bottom  level  of  a  mine  is  likely  to  continue  to  a 
considerably  greater  depth,  yet  may  not  feel  justified,  in  view  of 
the  hazardous  nature  of  any  mining  undertaking,  in  putting  a 
monetary  value  on  more  than  one-third  or  one-fourth  on  such 
tonnage. 


ORE  IN  SIGHT  107 

One  or  two  illustrations  may  serve  to  point  the  above  discus- 
sion. In  a  replacement  deposit,  where  the  lowest  workings  were 
only  about  250  ft.  below  the  outcrop,  and  the  examination  demon- 
strated the  existence  of  an  ore-shoot  1,700  ft.  long  and  averaging 
5  ft.  wide,  the  engineer  valuing  the  property  did  not  take  into  his 
ore  reserve  calculations  an  extension  of  more  than  50  ft.  below  the 
bottom  level.  This  he  called  probable  ore.  This,  no  doubt,  was 
quite  conservative,  especially  in  view  of  the  fact  that  the  indica- 
tions were  that  the  zone  of  maximum  enrichment  had  not  yet 
been  reached,  and  .there  was  every  likelihood  of  finding  higher- 
grade  ore  at  the  next  level.  Among  these  indications  may  be  noted 
that  the  bottom  level  of  the  mine  showed  better  values  than  any 
above  it.  Of  course,  the  question  in  all  these  forecasts  as  to  what 
an  orebody  will  do  is,  whether  the  same  length,  width  and  value  of 
ore-shoot  will  be  maintained. 

A  case  of  another  kind  is  the  estimation  of  probable  ore  in  a 
mine  like  the  Broken  Hill  South  Blocks  property,  Broken  Hill, 
New  South  Wales.  The  mine  adjoining  on  the  north  is  the  Broken 
Hill  South  mine,  which  has  been  opened  up  to  a  depth  of  more 
than  1,000  ft.  with  its  ore-shoots  pitching  towards  the  South 
Blocks  ground.  In  view  of  the  extensive  nature  of  the  under- 
ground workings  on  this  famous  lode  and  the  evidence  thereby 
afforded,  it  was  perfectly  justifiable  in  the  case  of  the  South  Blocks 
mine,  at  the  time  of  its  purchase  by  an  English  company  a  few  years 
ago,  to  assume  a  likelihood  of  a  continuance  of  the  ore  for  at  least 
300  ft.  below  the  then  bottom,  or  400-ft.  level.  Development  work 
since  then  has  proved  the  correctness  of  this  assumption. 

When  a  block  of  ground  is  opened  up  on  two  sides  as  by  a 
drift  and  a  winze,  it  is  usual  to  consider  half  the  rectangle  as  proved 
ore  and  to  take  it  into  the  tonnage  calculations  on  that  basis, 
namely,  the  quantity  included  in  the  triangle  made  by  the  develop- 
ment openings  and  the  straight  line  joining  their  ends.  The  size 
and  character  of  the  orebody,  and  the  extent  of  the  development 
must  to  a  considerable  extent  serve  as  a  guide.  In  a  mine  where 
only  a  meagre  amount  of  development  work  has  been  done  at 
shallow  depths  from  the  surface,  it  is  hazardous  to  make  any  great 
allowance  for  continuity  of  ore  beyond  the  existing  openings,  be- 
cause so  little  information  is  available.  On  the  other  hand,  it  may 
be  equally  risky  to  make  any  estimate  in  an  old  mine  of  consider- 
able depth,  despite  the  large  amount  of  evidence  available,  because 
of  the  likelihood  of  the  limits  of  the  profitable  ore  having  been 
reached. 

It  will  be  seen  from  the  foregoing  that  even  ore  opened  on  only 
two  sides  may  be  considered  proved  ore.  Probable  ore,  on  the 
other  hand,  may  be  exposed  only  on  one  side  or  only  at  one  point, 
that  is  to  say,  our  most  common  application  of  the  term  is  to  ore  be- 
low a  level,  or  beyond  any  development  opening,  where  the  geolog- 
ical evidence  affords  reasonable  proof  for  the  belief  that  the  ore 
will  extend  to  the  distance  assumed.  Prospective  ore  is  anything 


108  MINE    SAMPLING    AND   VALUING 

beyond  that.  Only  in  rare  cases  can  a  mine  be  purchased  at  a 
price  represented  by  the  net  profit  contained  in  the  proved  and 
probable  ore.  There  is  a  certain  amount  of  risk,  commonly  known 
as  mining  risk,  which  generally  must  be  assumed  in  the  purchase 
of  mining  property.  To  offset  this,  there  is  the  prospective  ore, 
which,  of  course,  is  the  likelihood  of  the  ore  continuing  to  a  point 
beyond  that  considered  safe  to  assess  in  the  ore  reserve  calcula- 
tions. The  likelihood  of  an  ore  deposit  continuing  in  depth  de- 
pends on  the  geological  conditions.  To  attempt  to  express  this  in 
definite  quantities  is,  as  Mr.  Hoover  states,  an  attempt  to  convey 
"an  impression  of  tangibility  to  a  nebulous  hazard."* 

What  Mr.  Hoover  means  undoubtedly  is,  that  this  pros- 
pective ore  cannot  be  stated  in  the  same  definite  terms  of  ton- 
nage as  are  the  proved  and  probable  ore.  However,  we  do  give 
it  a  definite  value  in  most  cases  by  paying  a  portion  of  the  pur- 
chase price  for  it,  namely,  the  likelihood  that  the  development 
will  prove  the  existence  of  the  additional  amount  of  ore  required 
to  return  the  capital  invested,  plus  a  reasonable  profit. 

If  we  were  dealing  with  a  deposit,  such  as  coal,  we  could 
assume  the  extension  of  the  coal  to  a  considerable  distance  be- 
yond the  existing  openings,  because  we  know  that  ordinarily  it 
occurs  with  a  regularity  unknown  in  metalliferous  deposits.  In 
making  such  an  assumption  the  risk  is  a  comparatively  small 
one  and  is  principally  restricted  to  geological  disturbances,  which, 
on  account  of  the  ease  with  which  they  can  be  determined  be- 
forehand, minimizes  such  risk.  The  Rand  banket  can  be  cal- 
culated with  a  considerable  amount  of  certainty,  although  the 
development  in  the  past  few  years  has  shown  that  practically  all 
the  former  estimates  as  to  the  likelihood  of  the  continuance  of 
ore  in  depth  have  been  wrong,  because  the  assay  of  the  ore  has 
decreased  greatly  as  depth  has  been  attained,  although  the  'reefs' 
are  as  strong  as  ever.  With  the  ordinary  type  of  mineral  deposit 
dealt  with  by  the  valuing  engineer,  the  data  available  does  not 
permit  of  a  prognostication  with  any  such  degree  of  certainty  as 
in  the  two  cases  here  cited,  therefore,  the  more  skillful  the  en- 
gineer, the  more  accurate  his  determination  of  the  prospective 
value  of  a  mine.  It  is  in  this  respect  we  find  some  of  the  great- 
est differences  between  the  reports  of  different  engineers  on  the 
same  property.  Aside  from  the  natural  conservatism  of  some 
men,  which  always  leads  them  to  make  an  estimation  as  safe  as 
possible,  a  great  deal  is  dependent  on  the  skill  of  the  engineer. 

No  rule  can  be  laid  down  as  to  what  properly  constitutes  pros- 
pective ore.  This  varies  in  each  individual  case,  and,  as  reiterated, 
is  dependent  on  the  geological  conditions.  If  there  was  any  cer- 
tainty of  the  prospective  ore,  it  would,  of  course,  be  classed  with 
either  proved  or  probable  ore,  but  in  view  of  its  uncertainty,  it 
must  be  expressed  in  indefinite  terms,  certainly  so  far  as  regards 
tonnage.  On  the  other  hand,  it  is  essential  for  the  valuing  engi- 

*'Principles   of  Mining,'   p.    18. 


ORE  IN  SIGHT  109 

neer  to  express  in  a  concrete  form  the  idea  of  prospective  value 
and  this  may  be  done  by  stating  that,  providing1  the  orebody 
maintains  the  same  dimensions  and  assay,  as  in  the  existing  work- 
ings, then  each  100  ft.  of  extension  in  depth  will  add  a  given  ton- 
nage to  the  available  ore  reserves.  Of  course,  this  may  be  expressed 
in  units  of  1  ft.,  but  a  unit  of  100  ft.  seems  preferable,  as  more 
nearly  approximating  working  conditions. 

Where  the  zone  of  secondary  enrichment  has  not  been  passed 
through,  it  is  hazardous  to  make  an  assumption  as  to  the  value, 
but  once  the  workings  have  penetrated  into  the  unaltered  ore, 
a  greater  degree  of  accuracy  is  likely  to  ensue  from  any  forecasts 
that  are  made.  In  another  chapter,  the  question  of  secondary 
enrichment  has  been  discussed  and  must  be  borne  in  mind  in 
connection  with  estimates  of  prospective  ore. 

Ore  Reserve  Plans. — From  the  assay  plan  the  limits  of  the 
proved  and  probable  ore  are  determined  and  the  mine  is  divided 
into  blocks,  the  size  and  shape  of  each  being  to  a  large  extent 
governed  by  the  mine  workings,  and  each  block  is  given  an  ap- 
propriate number  for  identification.  The  tonnage,  average  assay, 
and  dimensions  of  each  is  calculated  and  noted  on  a  special  plan 
known  as  the  ore  reserve  plan,  or  distinctive  colors  may  be  used 
to  represent  different  ranges  of  values,  so  that  the  approximate 
grade  of  each  block  is  seen  at  a  glance. 

Intermediate  between  the  assay  plan  and  the  ore  reserve  plan 
are  the  calculations  to  determine  the  tonnage  and  average  assay 
value  of  the  ore.  Generally  speaking,  ore  cannot  be  mined  as 
clean  as  it  can  be  sampled,  and  for  various  reasons,  fully  explained 
in  the  preceding  pages,  the  sampling  results  are  usually  higher 
than  the  breaking  value  of  the  ore.  It  is  customary,  therefore,  to 
make  certain  deductions  from  the  calculated  averages  as  obtained 
from  the  assay  plan  and  records,  and  this  takes  the  form  of  an 
arbitrary  reduction  in  grade,  which  may  amount  to  from  5  to  25%. 

Every  engineer  believes  his  samples  to  be  correct,  and  the 
necessity  for  reducing  the  assay  is  ascribed  to  the  fact  that  in 
mining,  a  certain  amount  of  the  country  rock  is  bound  to  become 
mixed  with  the  ore,  because  of  the  blasting  and  on  account  of 
the  natural  weakness  of  the  walls.  The  amount  of  wall  rock  that 
mixes  with  the  ore  and  dilutes  it,  depends  on  the  geological  con- 
ditions and  the  method  of  mining.  It  has  already  been  pointed 
out  that  on  the  Rand  the  stoping  value  of  the  ore  is  sometimes 
only  75%  of  the  sampling  results  and  in  other  districts  similar 
discrepancies  are  found.  As  a  usual  thing  no  account  is  taken  of 
the  increase  in  the  tonnage  on  this  account.  For  instance,  if  the 
ore  reserves  of  a  mine,  calculated  from  the  assay  plan,  amounted 
to  100,000  tons  of  an  average  value  of  $20,  then  if  a  deduction  of, 
say,  10%,  or  $2,  is  made  to  give  the  stoping  value,  there  results, 
according  to  general  practice,  100,000  tons  of  $18  ore. 

If  there  are  actually  100,000  tens  of  $20  ore  in  situ,  then  this 
is  incorrect  as  can  be  seen  by  calculating  the  total  gold  contents 


110  MINE    SAMPLING    AND   VALUING 

in  the  ore,  thus,  100,000  tons  at  $20  contains  $2,000,000.  After 
dilution  by  the  barren  wall  rock,  there  would  still  be  $2,000,000 
gold  in  the  ore,  but  its  average  value  as  broken  has  been  reduced 
to  $18.  If  we  divide  $18  per  ton  into  the  total  $2,000,000  in  the 
reserves,  there  results  111,111  tons,  so  that  it  is  assumed  11,111 
tons  of  waste  rock  will  get  mixed  with  the  ore,  allowing  the 
sampling  results  to  have  been  correct,  yet  the  extra  tonnage  is 
rarely  taken  into  account. 

Another  method  sometimes  followed  is  to  allow  for  the  ad- 
mixture of  the  10%  of  waste  rock.  This  gives  110,000  tons,  con- 
taining $2,000,000  of  gold,  which  by  calculation  shows  an  average 
assay  value  of  $18.17  per  ton.  By  this  method  advantage  is 
taken  of  the  whole  amount  of  gold  and  the  whole  of  the  calculated 
tonnage,  and  may  be  justifiable,  provided  the  ore  has  been  sampled 
properly,  and  the  samples  not  salted  by  the  brittle  mineral  in  the 
deposit. 

It  is  a  question  in  my  mind  whether  the  reason  that  the 
samples  assay  higher  than  the  broken  ore  is  not  in  most  cases  due 
as  much  to  unconscious  salting  of  the  samples  during  the  sampling 
operations  as  it  is  due  to  the  admixture  of  wall  rock.  One  rarely 
hears  of  a  mine  stoping  higher  than  the  sampling  results  and  a 
great  many  errors  are  made  at  this  stage  of  the  work,  as  generally 
too  little  allowance  is  made  and  the  ore  does  not  stope  to  the 
grade  assigned  to  it.  With  firm,  smooth  walls  less  deduction  need 
be  made  than  where  the  walls  are  bad,  or  the  ore  frozen  to  the 
walls,  or  where  values  shade  off  into  the  country. 

As  the  calculations  proceed,  the  various  blocks  of  ore  are 
tabulated,  each  under  its  appropriate  heading1,  with  the  number 
of  tons  in  each  block  and  its  average  value.  The  total  tonnage  in 
the  ore  reserves  is  obtained  by  adding  together  the  total  of  the 
proved  and  probable  ore.  The  average  value  is  determined  by 
the  same  method  as  used  in  averaging  assay  widths  and  values 
described  in  Chap.  XI  (page  83),  except  that  here  we  have  tons  instead 
of  a  unit  of  length.  Thus  there  ensues  a  certain  tonnage  of  proved 
and  probable  ore  of  a  calculated  average  value  from  which  the 
gross  metallic  contents  can  be  determined,  and  once  we  are  aware 
of  the  percentage  of  the  metallic  contents  that  can  be  recovered, 
and  the  cost  of  doing  so,  we  have  a  measure  of  the  value  of  the 
mine. 

With  base-metal  mines,  such  as  copper,  lead,  zinc,  tin,  etc.,  a 
basic  price  of  the  metal  is  sometimes  assumed  for  purposes  of  cal- 
culation. No  one  can  forecast  with  any  degree  of  certainty  the 
future  course  of  prices  of  metals ;  hence  any  assumption  as  to  price 
is  a  mere  gliess  that  may  be  wide  of  the  mark,  yet  in  order  to 
convey  a  concrete  impression  of  monetary  value  it  is  usual  to  as- 
sume a  price  for  the  metals. 


CHAPTER  XIII. 

CALCULATION  OF  PROFITS. 

Treatment  Problems. — The  profit  that  can  be  realized  from  any 
given  ore  with  any  given  market  price  for  metal  produced  depends 
on  the  metallurgical  treatment  and  the  working  costs.  Metallurgy 
has  no  place  in  this  discussion,  as  it  is  an  art  of  itself,  a  knowledge 
of  which  is  necessary  to  the  mine  valuer.  No  matter  how  care- 
fully a  mine  is  sampled  or  how  accurately  the  ore  reserves  are 
estimated,  the  whole  work  may  be  nullified  by  the  application  of 
the  wrong  method  of  treatment. 

As  a  usual  thing,  the  mine  valuer  is  not  an  expert  metallurgist, 
but  it  is  essential  for  him  to  have  a  firm  grasp  of  the  issues  involved 
and  at  least  sufficient  familiarity  with  the  subject  to  indicate  the 
correct  method  of  treatment  and  the  approximate  cost  thereof,  even 
if  he  may  not  be  able  to  carry  out  and  take  charge  of  the  practical 
working  of  the  method  itself.  No  rules  can  be  laid  down  for  the 
guidance  of  the  inexperienced — a  knowledge  of  metallurgy  is  essen- 
tial and  each  problem  must  have  its  own  solution.  The  subjects 
on  which  the  modern  engineer  is  called  to  pass  judgment  are  so 
varied  that  no  man  can  be  expert  in  all  and  it  is  a  pity  that  more 
engineers  do  not  recognize  their  limitations  in  regard  to  metallurgy, 
and  before  committing  their  clients  to  a  heavy  expenditure  for  plant, 
call  in  the  metallurgist  to  direct  them. 

An  analysis  of  the  ore  to  be  treated  is  not  always  a  sufficient 
guide  in  deciding  ori  the  treatment  and,  wherever  possible,  large 
scale  tests  should  be  made  to  demonstrate  the  efficacy  of  the  method 
to  be  used.  Such  tests  will  show  the  metallurgical  losses  and  the 
percentage  of  the  gross  value  that  will  be  recovered  in  a  marketable 
form.  If  a  mixed  lead-zinc  ore  is  to  be  treated  and  each  can  be 
recovered  as  a  separate  product,  the  amount  of  such  products  must 
be  determined  and  a  knowledge  of  market  conditions  is  necessary 
to  enable  the  net  value  of  each  to  be  arrived  at.  Enough  has  been 
said  to  show  that  the  gross  metal  contents  of  an  ore  is  not  neces- 
sarily an  index  of  the  net  value  and  more  especially  is  this  true  with 
ores  consisting  of  a  mixture  of  base  metals,  such  as  lead,  zinc,  and 
copper,  with  precious  metals. 

Working  Costs. — The  method  of  treatment  having  been  decided 
on,  the  working  costs  can  be  determined.  Through  lack  of  skill  and 
experience  more  errors  are  made  in  estimating  costs  than  in  any 
other  branch  of  mine  valuation,  and  more  companies  come  to  grief 
on  account  of  under-estimated  working  costs  than  from  over- 
estimated ore  reserves.  The  short  time  an  engineer  spends  on  a 
mine  examination  in  many  instances  enables  him  to  obtain  little 
accurate  knowledge  of  the  economic  conditions  of  the  district  and 
an  ample  factor  of  safety  must  usually  be  allowed  if  serious  errors 
are  to  be  avoided.  When  data  as  to  the  costs  at  neighboring  mines 
is  available  the  problem  is  greatly  simplified,  but  failing*  this,  the 


112  MINE   SAMPLING   AND   VALUING 

engineer  has  only  his  experience  to  guide  him,  and  in  remote  dis- 
tricts the  problem  may  become  one  of  great  difficulty.  It  should 
be  resolutely  faced  and,  while  care  is  advocated,  it  must  not  be 
carried  to  such  extremes  as  to  stifle  a  legitimate  business. 

The  net  value  of  the  ore,  or  profit,  is  the  difference  between  the 
cost  of  producing  it  and  the  actual  net  sum  received  from  the  sale 
of  the  metals  or  minerals  recovered  from  it.  What  properly  con- 
stitutes costs  of  operation  is  a  question  of  accountancy,  but  from 
a  valuation  standpoint,  it  may  be  stated  that  whatever  temporary 
disposition  may  be  made  of  certain  items  of  expenditure,  when  the 
mine  is  exhausted,  the  cost  of  production  is  equal  to  the  difference 
between  the  amount  realized  from  the  sale  of  the  products  and  the 
dividends  distributed. 

In  the  May,  June  and  July,  1912,  issues  of  the  Mining  Magazine, 
T.  A.  Rickard,  in  a  series  of  articles,  entitled  'Phantom  Profits,'  has 
drawn  attention  to  the  misconception  that  may  arise  from  accepting 
ordinary  statements  denominated  operating  cost  or  working  cost. 
Certain  items  of  expenditure,  such  as  extra  remuneration  to 
directors,  cost  of  underwriting,  head  office  expenses,  and  cost  of 
extra  equipment  required  by  advances  in  metallurgy,  etc.,  are  not 
shown  in  the  cost  sheets  but  are  allocated  in  the  annual  accounts. 
On  the  average  these  amount  to  20%  of  the  so-called  operating 
cost.  The  instances  quoted  in  the  articles  are  all  from  operating 
companies,  but  they  are  equally  important  to  the  valuing  engineer. 
Unfortunately,  mine  valuers  make  too  little  allowance  for  expense 
of  that  character. 

The  tendency  during  the  past  two  or  three  decades  has  been 
for  the  price  of  all  commodities,  including  labor,  to  go  up.  This  is 
often  overlooked  and  may  lead  to  disastrous  results.  In  new  dis- 
tricts with  a  limited  labor  supply  the  starting  of  operations  on  a 
large  scale  may  within  a  short  time  cause  a  serious  rise  in  wages 
and  thereby  adversely  affect  working  costs.  These  things  must 
not  be  overlooked,  nor  must  the  likelihood  of  costs  going  up  as 
greater  depth  is  attained,  on  account  of  heavier  pumping  and 
winding  charges.  Another  variable  factor  is  that  which  may  arise 
from  variations  in  freight  rates  and  the  charges  of  customs  smelters, 
where  the  output  of  a  mine  is  shipped  either  as  mined  or  in  the 
form  of  concentrate.  Freight  rates  may  have  a  serious  influence  on 
costs  where  an  ore  is  smelted  locally  and  fuel  or  flux,  or  both, 
required  to  be  brought  from  a  distance. 

Allowance  should  always  be  made  for  bad  management,  as  the 
attainment  of  estimated  working  costs  may  be  entirely  dependent 
on  highly  efficient  management.  The  cost  of  construction  and  de- 
velopment, as  well  as  amortization  of  capital  on  the  scale  of  opera- 
tions proposed,  can  be  fairly  accurately  established  by  the  experi- 
enced engineer.  Where  he  is  likely  to  fail  is  in  connection  with 
those  items  which  are  purely  of  a  head  office  character  and  which 
are  entirely  beyond  his  control. 

Base  Metal  Prices. — In  arriving  at  the  value  of  any  metal  mine, 
except  gold,  the  selling  price  of  the  metal  must  be  taken  into  ac- 


CALCULATION    OF    PROFITS 


113 


count.  It  is  well  known  that  the  price  of  all  the  metals  is  subject 
to  wide  fluctuations,  dependent  on  the  law  of  supply  and  demand 
and  other  economic  conditions,  needless  to  discuss  here.  A  study 
of  the  statistics  is  no  guide  to  the  future  price  and  those  most 
familiar  with  the  business  are  just  as  likely  to  arrive  at  an  incorrect 
figure  as  less  experienced  persons. 

Mr.  Hoover,*  although  recognizing  the  hazard  in  doing  so, 
estimated  in  1909  that  from  the  available  outlook  the  following 
would  be  the  normal  prices  of  various  metals  for  "some  time  to 
come":  Lead  Spelter  Copper  Tin  Silver 

London  £  per  ton  13.5  21.  65  130  26d.  per  oz. 

New  York  cents  per  Ib.     4.3  5.0  14.  0.29  52c.    " 

This  estimate  was  made  as  the  result  of  a  careful  study  of  the 
statistical  position  of  the  various  metals  combined  with  an  analysis 
of  the  general  trade  conditions  of  the  world,  which  naturally  govern 
the  demand  for  commodities  of  all  kinds. 

Despite  Mr.  Hoover's  great  ability  and  experience  he  failed  to 
gauge  actual  metal  prices  in  the  four  years  which  have  elapsed 
for  which  the  statistics  are  available.  No  man  can  successfully 
look  any  distance  into  the  future.  The  demand  for  metals  depends 
on  trade  activity  and  trade  activity  is  governed  by  a  number  of 
factors,  many  of  which  are  unforeseen,  or  even  beyond  control  of 
man.  Good  harvests  in  various  parts  of  the  world  will  cause  an 
expansion  of  all  sorts  of  business,  people  have  more  money  to  spend, 
new  construction  is  stimulated,  new  buildings,  new  railroads  and  other 
industrial  enterprises  are  undertaken  requiring  larger  amounts  of 
the  metals.  On  the  other  hand,  drought  or  pestilence  may  have  the 
opposite  effect,  unexpected  political  disturbances  may  restrict  trade, 
which  naturally  affects  metal  prices  in  proportion.  For  this  reason 
any  forecast  as  to  the  future  price  of  the  metals,  beyond  perhaps 
a  few  months,  is  liable  to  be  falsified  by  the  attained  figures. 

The  actual  London  prices  per  ton  of  the  various  metals  for 
1909,  the  year  of  Mr.  Hoover's  estimate,  and  the  three  succeeding 
years,  is  as  follows : 


Lead 
L.     s.     d. 

Spelter 
L.     s.     d. 

Copper 
L.     s.     d. 

Tin 
L.     s.     d. 

Silver  per  oz. 
d. 

Hoover  

13/  5/0 

21/  0/0 

65  /  0/0 

130/  0/0 

26.0000 

1909  

13/  1/8 

221  3/0 

58/17/3 

134/15/6 

23.7217 

1910  

12/19/0 

23/  0/0 

57/  3/2 

155/  6/2 

24.7085 

1911  

13/19/2 

25  /  3/2 

56/  1/9 

192  /  7/1 

24.6531 

1912  

17/  9/6 

26/10/0 

72/18/1 

209/  8/5 

28.0349 

The  difference  between  Mr.  Hoover's  estimate  and  the  actual  average   metal 
prices  is  interesting. 

J.  R.  Finlay,  employed  by  the  state  of  Michigan,  U.  S.  A.,  to 
value  the  copper  mines  of  the  state  for  taxation  purposes  a  few  years 

* 'Principles   of   Mining,1    page    37. 


114 


MINE    SAMPLING   AND   VALUING 


ago,  assumed  as  a  basis  for  calculation,  copper  at  14c.  per  Ib.  In 
a  recent  utterance  he  admits  a  change  of  opinion  and  now  believes 
15c.  will  be  nearer  the  average  price  in  the  future.  These  two  in- 
stances are  quoted  with  a  view  to  showing  how  difficult  it  is  for  even 
those  familiar  with  the  subject  to  make  any  forecast  as  to  future  prices. 

With  the  large  percentage  increase  in  the  consumption  of  metals 
during  the  past  few  years  it  would  look  as  if  a  great  scarcity  of 
copper  must  ensue.  No  new  mines  are  being  opened  or  equipped 
in  the  United  States  and  every  likely  district  has  been  thoroughly 
prospected.  Elsewhere  a  similar  condition  exists,  so  that  those 
most  familiar  with  the  copper  industry  predict  higher  prices  for  the 
red  metal.  The  last  eighteen  months  has  brought  to  the  front  at 
least  one  great  mine,  namely,  the  Chuquicamata  in  Chile,  but  more 
important  still  is  the  successful  application  of  the  flotation  methods 
for  the  treatment  of  copper  ores.  Tests  so  far  made  indicate  an  in- 
creased extraction  of  from  20  to  30%  over  that  attained  by  water 
concentration.  The  introduction  of  this  process  in  mines,  like  the 
Utah  Copper,  Nevada  Consolidated,  or  other  great  producers,  will  im- 
mediately add  considerable  quantities  of  copper  to  the  existing  outputs. 

The  flotation  process  is  applicable  to  a  wide  range  of  ores,  and 
just  as  the  cyanide  process  revolutionized  the  treatment  of  gold 
ores,  so  does  flotation  threaten  to  render  available  great  quantities 
of  material  that  heretofore  would  not  lend  itself  to  profitable 
handling1.  The  history  of  the  world  shows  that  as  the  demand 
for  a  thing  has  arisen,  the  ingenuity  of  man  has  found  a  source 
of  supply  to  meet  it.  Other  factors  of  a  similar  nature  affect  the 
other  metals  and  render  a  prognostication  of  their  future  prices 
difficult  in  the  extreme. 

An  interesting  table  showing  the  'Increase  of  World's  Pro- 
duction of  Metals,'  prepared  by  Bedford  McNeill,*  is  well  worthy 
of  study  in  this  connection  as  emphasizing  this  difficulty: 

INCREASE  OF  WORLD'S  PRODUCTION  OF  METALS. 


Percentage 

1889. 

1891. 

1901. 

1911. 

Increase  for 
Ten  Years 

Ending  1911. 

Tons 

Tons 

Tons 

Tons 

Pig  Iron  

41,000,000 

65,000,000 

58 

Copper.  . 

261,205 

279,391 

526,000 

884,000 

68 

Zinc. 

329  600 

356  200 

500,000 

900,000 

80 

Lead 

540  200 

589  000 

850,000 

1,100,000 

29 

Tin 

55  400 

59  500 

88,000 

116,000 

32 

Nickel 

1  800 

4,700 

9,000 

24,000 

144 

Aluminum  .... 

70 

328 

7,500 

46,000 

513 

Mercury. 

3,700 

3,700 

3,000 

4,000 

33 

Silver 

4,100 

4,700 

5,300 

7,500 

41 

Gold  

380 

680 

79 

Antimony  

10,000 

23,000 

130 

The   increased   output   of   nickel,    aluminum,    and   antimony   is 
striking  and  is  due  to  lower  cost  of  manufacture  and  new  uses  for 


Presidential   Address,   Inst.   Min.    and   Met.,   March,    1913. 


CALCULATION    OF    PROFITS  115 

those  metals  resulting  therefrom.  The  increment  with  the  more 
commonly  used  metals,  while  not  so  large  in  percentage,  has  been 
stupendous  in  point  of  actual  quantity,  namely,  pig  iron  by 
24,000,000  tons;  copper,  358,000  tons;  zinc,  400,000  tons,  and  lead, 
250,000  tons.  This  represents  in  the  aggregate  a  remarkable  in- 
crease in  the  world's  purchasing  power,  because  taken  over  a 
period  of  years,  prices  have  not  fallen,  but  have,  on  the  whole, 
risen.  Within  the  ten-year  periods  themselves  there  have  been 
fluctuations  between  wide  extremes,  and  the  average  prices  for 
individual  years,  when  compared,  bring  home  in  a  startling 
manner  the  necessity  of  making  a  forecast  for  the  immediate 
future  with  some  degree  of  accuracy.  The  average  price  over 
a  term  of  years  must  be  the  guide  in  estimating  profits,  but  the 
fact  must  not  be  overlooked  that  a  few  years  of  low  metal  prices 
may  seriously  cripple  the  finances  of  a  company,  especially  in 
its  earlier  life,  and  may  even  be  the  cause  of  its  bankruptcy. 

It  is  not  within  the  purpose  of  this  book,  nor  is  it  essential, 
to  fathom  the  economic  causes  underlying  the  increased  cost  of 
many  commodities  during  the  past  few  years ;  nevertheless,  com- 
modities do  cost  more,  and  safe  to  say  no  one  could  have  forecast 
the  percentage  increase  with  any  degree  of  accuracy.  The  en- 
gineer can  only  meet  the  problem  that  confronts  him  by  using 
the  evidence  at  hand  and  must  not  endeavor  to  peer  too  deeply  into 
the  dim  haze  of  the  future.  A  normal  price  of  the  metal  must  be 
assumed  and,  in  view  of  the  difficulty  involved,  it  is  better  to  be 
conservative  rather  than  excessively  optimistic. 

During  periods  of  high  metal  prices  the  average  human  being 
is  more  inclined  to  take  an  optimistic  view  of  such  matters  than 
during  a  time  of  depression.  One  factor  that  is  rarely  taken  into 
account  is,  that  when  metals  are  selling  at  high  prices,  giving 
greatly  increased  profits,  development  work  is  accelerated  and 
other  extraordinary  expenditures  undertaken,  which,  when  metal 
prices  are  low,  are  entirely  suspended. 

The  normal  price  or  average  price  represents  the  mean  be- 
tween the  high  and  low  levels.  Mr.  Hoover  sugests  the  idea  that 
on  the  low  swing  metals  reach  a  level  below  which  the  price  will 
not  fall  and  this  he  calls  the  basic  price.  It  seems  to  me  to  be 
too  difficult  to  determine  such  basic  prices,  and  they  must  vary 
from  time  to  time,  depending*  on  the  law  of  supply  and  demand 
and  other  economic  factors.  One  such  factor  on  which  the  en- 
gineer cannot  reckon  with  any  degree  of  certainty  is  the  result 
of  those  combinations  of  business  interests  known  as  trusts,  pools, 
or  conventions,  which  have  for  their  object  the  regulation  of 
prices.  It  is  well  known,  for  instance,  that  the  price  of  spelter  is 
artificially  maintained,  as  is  the  price  of  antimony  and  some  of  the 
others  to  a  lesser  degree.  With  prices  artificially  regulated,  wide 
fluctuations  are  less  likely  during  the  maintenance  of  the  com- 
bination, but  its  dissolution,  by  permitting  or  even  fostering  com- 
petition, may  result  in  prices  below  their  legitimate  level  until 
accumulated  stocks  are  exhausted. 


CHAPTER  XIV. 

AMORTIZATION  OF  CAPITAL. 

In  the  abstract,  the  total  net  profit  contained  in  the  ore  re- 
serves of  a  mine  on  a  fixed  basis  for  cost  of  production  and  prices 
realized  by  the  metals  produced,  remains  the  same,  regardless  of 
the  length  of  time  it  takes  to  realize  that  profit.  On  the  other 
hand,  the  demands  of  finance  require  the  consideration  of  interest 
rate  on  capital  invested  and,  therefore,  the  time  it  takes  to  realize 
the  profit  in  the  ore  reserves.  This  is  governed  by  the  number 
of  tons  per  annum  mined  and  treated. 

Where  interest  rates  are  low,  there  is  not  the  same  necessity 
for  intensity  of  production  as  where  they  are  high.  With  a  given 
amount  of  capital  invested,  the  rate  of  production  may  be  so  low 
as  to  cause  operations  to  be  carried  on  at  a  loss,  or  at  least  not 
yield  sufficient  profit  to  pay  interest  on  the  investment,  much  less 
provide  for  a  return  of  such  capital.  Increase  the  annual  output 
sufficiently  and  the  earnings  increase  to  the  extent  required.  This 
question  of  increase  of  the  rate  'of  production  is  closely  inter- 
woven with  the  size  and  capacity  of  the  plant,  and  this  brings  in 
an  additional  financial  factor,  namely,  the  cost  of  such  larger 
plant,  which  capital  outlay  in  its  turn  must  be  amortized  and 
return  a  suitable  rate  of  interest.  The  plant  may  be  increased 
beyond  the  capacity  of  the  mine  and,  although  the  annual  return 
may  temporarily  increase,  the  net  result  shows  a  smaller  total 
than  would  have  been  achieved  with  a  smaller  plant. 

The  scale  of  operations  necessary  to  afford  an  adequate  return 
on  the  capital  invested  will  vary  with  each  different  case,  and  the 
engineer  must  judge  each  one  on  its  merits.  A  2%  copper  ore 
under  the.  conditions  prevailing  in  most  parts  of  the  world  can 
only  be  profitably  handled  on  a  scale  large  enough  to  warrant  the 
expenditure  of  capital  necessary  to  install  labor-saving  appliances 
of  all  kinds  and  the  equipment  required  to  secure  low  working 
costs.  Unless  the  deposit  to  be  exploited  is  capable  of  yielding 
the  required  tonnages  during  a  period  sufficiently  long  to  return 
such  capital  invested,  plus  a  satisfactory  rate  of  interest,  it  cannot 
be  considered  commercially  valuable. 

The  porphyry  copper  mines,  such  as  the  Utah  Copper,  Nevada 
Consolidated,  Chino  and  others,  where  the  copper  occurs  in  a 
silicious  gangue  which  requires  to  be  separated  from  the  valuable 
mineral  by  concentration,  and  the  low-grade  gold  mines,  such  as 
the  Alaska  Treadwell  and  Homestake,  are  worked  at  a  profit  only 
because  of  the  large  tonnage  handled.  Were  an  attempt  made 
to  work  these  same  mines  with  an  output  of,  say  100  tons  of  ore 
per  day,  the  inevitable  result  would  be  loss.  The  engineer  must 
take  this  factor  into  his  calculations  as  seriously  influencing  re- 
turn of  capital  and  interest  on  investment. 


AMORTIZATION   OF   CAPITAL  117 

All  capital  is  invested  with  the  expectation  that  it  will  be  re- 
turned together  with  a  suitable  rate  of  interest  thereon.  There  is 
a  wide  gap  between  the  methods  employed,  for  instance,  by  the 
actuaries  of  a  life  insurance  company  in  valuing  its  investments 
and  those  applied  to  mining.  In  the  purchase  of  so-called  gilt- 
edged  securities,  such  as  government  bonds,  a  definite  rate  of 
interest,  plus  a  return  of  the  capital,  can  be  easily  calculated,  and 
there  is  a  practical  assurance  that  such  figure  will  be  realized; 
the  sole  risk  being  the  failure  of  the  particular  government  to  meet 
its  obligations. 

When  we  come  to  industrial  enterprises  a  certain  amount  of 
risk  is  involved,  due  to  the  fact  that  the  yield  is  more  or  less 
dependent  on  personal  equation,  varying  economic  conditions  and 
other  undetermined  factors,  which  may  influence  the  eventual 
profit.  So  far  as  the  capital  invested  in  such  business  is  concerned, 
this  is  originally  required  for  the  purpose  of  equipping  the  business 
for  operation,  the  purchase  of  plant  and  equipment,  and  so  forth. 
The  life  of  industrial  enterprises  is,  in  most  cases,  more  or  less 
unlimited,  depending  on  the  ability  of  each  individual  business 
to  continue  the  struggle  for  existence  by  meeting  competition,  etc. 
Once  the  original  capital  outlay  for  plant  and  equipment  is  made, 
its  cost  can  be  amortized  at  a  definite  rate  and,  providing  a  suitable 
allowance  is  made  for  upkeep  and  renewals,  the  entire  capital  ac- 
count can,  if  necessary,  be  liquidated  in  a  predetermined  number  of 
years. 

In  making  an  investment  in  an  industrial  enterprise,  the  re- 
turns can  be  calculated  with  the  same  exactness  as  with  gilt- 
edged  securities,  except  that  there  is  a  greater  risk  involved,  due 
to  the  inherent  nature  of  the  business.  On  the  other  hand,  with 
a  mine,  every  ton  of  ore  that  is  extracted  diminishes  by  a  definite 
amount  the  assets  of  the  business  and  brings  the  property  so  much 
nearer  to  exhaustion.  An  expansion  of  a  mining  enterprise  is 
only  a  comparative  statement  and  does  not  detract  in  any  way 
from  the  fundamental  fact,  that  with  the  exhaustion  of  the  ore  the 
business  itself  must  cease.  It  is  in  this  respect  that  mining  differs 
from  all  other  business  and  for  that  reason  a  mining  investment 
must  be  viewed  from  an  entirely  different  standpoint  from  the 
others.  A  great  deal  has  been  written  about  amortization  of 
capital  in  mining  enterprises,  but  its  usefulness,  except  in  a  few 
special  cases,  is  questionable. 

It  will  appear  from  the  foregoing,  that  in  order  to  be  able  to 
reckon  on  amortization,  it  is  necessary  to  have  a  more  or  less 
assured  definite  period  during  which  the  capital  can  be  retired. 
In  other  words,  the  business  must  be  assumed  to  continue  for  a 
sufficient  number  of  years  to  permit  of  this.  As  has  already  been 
indicated,  it  is  rarely  possible  to  buy  a  mine  where  the  net  profit 
in  the  ore  reserves  is  equal  to  the  purchase  price.  The  mine  is 
bought  on  the  basis  of  the  proved,  probable  and  prospective  ore, 
and  according  to  the  amount  of  the  purchase  price  represented  by 


118  MINE    SAMPLING   AND   VALUING 

such  prospective  ore,  to  that  extent  does  the  element  of  specula- 
tion enter  into  the  investment.  Hence  the  difficulty  of  applying 
the  rules  of  amortization  to  a  mining  enterprise.  With  such 
properties  as  the  Rio  Tinto,  some  of  the  porphyry  coppers,  the 
large  Rand  amalgamations  and  a  few  others,  where  the  ore  re- 
serves are  enormous,  amortization  tables  may  be  applied,  but  in 
the  vast  majority  of  mines  it  is  out  of  the  question. 

The  theory  of  redemption  or  amortization  of  capital,  according 
to  Mr.  Hoover,  is  that :  "A  portion  of  the  annual  earnings  must 
be  set  aside  in  such  a  manner  that  when  the  mine  is  exhausted  the 
original  investment  will  have  been  restored."1  This  further  in- 
volves that  these  annual  instalments  are  considered  as  payments 
before  the  due  date  and,  if  placed  at  compound  interest,  will  re- 
deem the  capital  at  the  end  of  the  period  which  in  a  mining  enter- 
prise would  correspond  to  the  time  when  the  mine  is  exhausted. 
But  Mr.  Hoover  observes  that :  "In  the  practical  conduct  of 
mines  or  mining  companies,  sinking  funds  for  amortization  of 
capital  are  never  established."2  The  reason  for  this  has  already 
been  given.  I  do  not  mean  to  convey  the  idea  that  amortization 
is  not  to  be  taken  into  consideration  in  valuing1  a  mining  property, 
but  its  real  usefulness  must  be  to  serve  as  a  guide  to  indicate  the 
amount  of  risk  involved.  The  rate  of  production  naturally  gov- 
erns the  annual  income,  and  from  this  the  number  of  years  during 
which  this  must  continue  in  order  to  amortize  the  capital  and  pay 
a  suitable  rate  of  interest  thereon  can  easily  be  computed. 

If  the  life  of  the  mine,  as  determined  by  the  proved  and  prob- 
able ore,  is  less  than  such  term  of  years,  then  the  difference  is 
equal  to  the  risk  involved  in  making  the  purchase.  On  top  of  this 
the  question  naturally  arises  as  to  the  amount  of  risk  that  it  is 
advisable  to  allow  in  a  mining  investment.  On  this  subject,  too, 
a  great  deal  has  been  written,  but  it  cannot  be  indicated  by  any 
formula  or  any  amount  of  printed  matter.  It  must  be  decided  by 
the  engineer  as  the  result  of  his  study  of  the  deposit  and  the  known 
economic  conditions.  This  involves  broad  questions  of  experience 
and  judgment.  A  proper  estimate  of  the  geology  of  the  deposit 
will  permit  of  a  forecast  as  to  extension  in  depth,  while  the  economic 
conditions  involve  such  questions  as  varying  metal  prices,  cost 
of  labor,  supplies  and  the  like. 

In  the  ordinary  mining  project  that  comes  within  the  engineer's 
purview,  there  is  rarely  more  than  three  or  four  years'  ore  in 
sight,  and  at  the  known  rates  of  profit  resulting  from  the  operations 
at  such  mines,  it  becomes  impossible  to  amortize  the  capital  except 
in  the  manner  already  indicated,  namely,  by  additions  to  the  ore 
reserves  or  increased  metal  prices.  If  a  mine  can  be  purchased  on 
a  basis  showing  a  full  return  of  the  capital,  plus  interest  thereon, 
it  would  be  an  extremely  desirable  condition ;  unfortunately,  how- 
ever, this  is  rarely  possible  and  we  are,  therefore,  compelled  to  pay 
a  portion  of  the  purchase  price  for  a  possibility.  In  other  words, 

^'Principles   of   Mining,'  Page  42. 
2<Principles   of  Mining,'  Page  42. 


AMORTIZATION   OF   CAPITAL  119 

we  cannot  ..avoid  the  element  of  risk,  which  is  almost  inseparable 
from  any  mining  transaction,  and  as  Mr.  Bedford  McNeill  states  :* 
"It  is  no  use  ignoring  or  pretending  to  ignore  the  fundamental 
fact  that  mining  is  and  must  always  continue  to  be  essentially 
speculative."  Therefore,  it  devolves  upon  the  mining  engineer  to 
confine  the  risk  within  reasonable  limits. 

There  are  various  ways  of  expressing  the  percentage  of  risk 
one  is  justified  in  taking.  I  believe  J.  H.  Curie  advocates  that  if 
the  profit  contained  in  the  ore  in  sight  is  equal  to  75%  of  the  pur- 
chase price  and  the  bottom  of  the  mine  is  g'ood,  the  purchase  is 
justified.  Others  writers  advocate  a  stated  rate  of  interest  per 
annum,  but  the  application  of  any  such  rule  is  dependent  on  the 
particular  conditions  involved  and  attempting  to  make  a  law  that 
can  be  generally  applied  is  like  trying  to  move  a  ship  without 
raising  the  anchor — it  is  perfectly  safe,  but  the  ship  will  never 
get  anywhere.  Business  acumen  must  remain  the  guide  to  what 
percentage  of  risk  may  be  taken  in  any  particular  case- 


*Presidential   Address,    lust.   Min.    and  Met.,   March,    1913. 


CHAPTER  XV. 

WRITING  REPORTS. 

Every  engineer  has  his  own  method  of  setting  out  the  facts  gath- 
ered in  the  course  of  his  examination  of  a  property,  but  nowhere  is 
brevity  more  to  be  commended.  In  general,  it  may  be  stated  that  the 
report  should  give  such  data  that  another  engineer,  willing  to  assume 
the  sampling  as  having  been  properly  done,  will  be  enabled  to  check 
the  results  and  form  his  own  opinion  as  to  the  value  of  the  property 
in  question. 

It  often  happens,  especially  with  younger  engineers,  ambitious  to 
show  the  painstaking  manner  in  which  they  have  carried  out  their 
work,  that  the  report  is  padded  with  verbose  descriptions  and  discus- 
sions of  useless  topics.  Instead,  it  should  be  confined  strictly  to  such 
facts  as  influence  the  value  of  the  mine,  and  all  other  topics,  even  those 
relating  to  mine  management,  had  generally  better  be  omitted,  as  it 
not  only  unnecessarily  lengthens  the  report,  but  further  tends  to  fog 
the  vital  issue.  At  times,  it  may  be  necessary  to  discuss  managerial 
policy  as  affecting  the  profits  to  be  derived  from  the  business,  but  in 
nearly  all  cases  this  can  better  be  discussed  in  an  appendix. 

There  have  been  a  number  of  report  forms  published  giving  the 
various  headings  and  the  order  in  which  the  topics  entering  into  a 
mine  report  should  be  discussed.  These  are  useful  to  some  extent, 
especially  to  the  younger  engineer,  but  they  must  all  be  modified  to 
meet  the  special  conditions  of  each  case. 

The  report  should  be  as  brief  as  consistent  with  an  exposition  of 
the  facts  and  the  topics  should  be  taken  up  in  logical  sequence.  One 
of  the  most  common  errors  is,  in  the  discussion  of  the  geology,  to  an- 
ticipate the  assay  value  of  the  ore  or  the  tonnage  available.  No  sub- 
ject should  be  introduced  until  its  proper  place.  Many  reports  con- 
tain pages  of  descriptive  matter  relating  to  the  topography  of  the 
country,  roads,  water  supply,  etc.,  placed  before  the  discussion  of  the 
mine  workings  proper.  In  most  cases  these  should  be  added  as  an 
appendix  to  the  main  report,  or  such  as  influence  the  working  costs 
should  immediately  precede  the  chapter  dealing  with  that  subject. 

I  recall  a  report  written  by  a  prominent  engineer  that  consisted 
of  more  than  100  typewritten  quarto  pages  of  descriptive  matter,  that 
concluded  by  stating  there  were  only  10,000  tons  of  4  %  copper  ore 
available  and  the  mine  was  not  worthy  of  consideration.  The  prop- 
erty was  situated  in .  a  remote  district  in  the  tropics,  where  skilled 
labor  was  not  to  be  had  and  the  other  economic  conditions  adverse. 
The  writer  of  the  report  described  the  country,  its  fauna  and  flora, 
the  economic  conditions,  questions  of  climate,  transport,  labor  and 
other  things  that  might  have  an  influence  on  a  going  concern,  but 
were  quite  useless  in  view  of  the  fact  that  the  property  was  not  rec- 


WRITING  REPORTS  121 

ommended.  All  these  things  matter  not  a  whit  if  the  result  of  the 
examination  shows  the  property  to  be  of  no  present  value  or  any 
likelihood  of  being  so  in  the  future. 

Some  engineers  recommend  that  a  report  should  open  with  a 
summary.  This  is  illogical ;  the  proper  place  for  the  summary  and 
recommendations  is  at  the  end,  where  it  is  just  as  easily  found.  A 
report  should  be  an  impersonal  document,  and,  as  pointed  out  in 
Chapter  I,  should  be  an  impartial  exposition  of  the  facts  regardless 
of  whether  the  engineer  is  acting  for  a  buyer  or  seller.  The  engineer 
must  not  act  as  an  advocate.  Sometimes  reports  are  used  at  a  later 
date  for  a  different  purpose  than  originally  intended.  As  a  matter 
of  personal  protection  and  to  prevent  misconception  on  the  part  of 
the  reader,  and  to  prevent  improper  use  being  made  of  it  the  engi- 
neer should  at  the  beginning  of  the  report  definitely  state  the  object 
of  the  report  and  its  scope. 

The  basis  of  all  calculation  of  estimates  should  be  definitely  stated 
in  unmistakable  and  concise  language.  The  conclusions  arrived  at 
should  likewise  be  definite.  The  engineer  is  called  upon  to  give  an 
opinion  as  to  the  value  of  the  property  and  such  opinion  should  be 
stated  in  unequivocal  language.  The  custom  is  only  too  prevalent 
of  qualifying  recommendations  in  such  a  manner  as  to  rob  them  al- 
most entirely  of  their  positiveness.  The  engineer's  client  has  the 
right  to  expect  a  definite  recommendation  as  much  as  a  patient  has  a 
right  to  expect  a  remedy  from  his  physician. 

In  foreign  countries  where  a  different  system  of  weights,  measures 
and  currency  is  used,  engineers  are  prone  to  employ  both  the  expres- 
sions used  in  the  foreign  country  and  their  own  in  the  same  report, 
even  on  the  same  page,  so  that  considerable  confusion  may  arise  in 
the  mind  of  the  reader  therefrom.  For  instance  in  a  place  like 
Mexico,  where  Mexican  weights  and  measures  are  used,  as  also  the 
metric  system,  together  with  Mexican  dollars  and  gold  dollars,  a 
great  deal  of  confusion  may  arise  unless  the  same  nomenclature  is 
used  throughout.  Reports  on  Russian  properties  often  lead  to  a 
great  deal  of  confusion,  because  the  engineer  will  use  the  Russian 
system  in  places  and  the  English  in  others.  A  proper  report  should 
not  only  be  built  up  logically,  but  the  greatest  effort  should  be  made 
to  convey  a  clear  meaning.  Engineers  as  a  class  lack  literary  ability, 
but  this  quality  can  be  cultivated  as  well  as  any  other. 

The  engineer  is  sometimes  called  upon  to  compile  a  report  from 
second-hand  data,  that  is,  data  which  he  has  not  himself  gathered. 
This  proceeding  is  very  often  fraught  with  a  considerable  amount  of 
danger  and  may  easily  lead  to  serious  mistakes  being  made.  Nat- 
urally the  basis  for  such  a  report  must  be  the  assumption  that  the 
information  is  correct.  Very  often  this  is  not  the  case.  No  doubt 
the  future  operations  of  a  mine  are  largely  dependent  on  the  quan- 
tity and  grade  of  its  ore  reserves,  and  given  a  record  of  past  opera- 
tions one  is  generally  justified  in  assuming  a  continuance  of  previ- 
ously attained  figures.  In  the  estimation  of  ore  reserves  by  the 
management  of  a  mine  the  personal  element  enters  more  largely 


122  MINE    SAMPLING   AND   VALUING 

than  it  does  in  mine  valuation  work  per  se.  In  other  words  the 
mine  valuer  brings  to  bear  a  more  judicial  frame  of  mind  than  does 
the  mine  management,  who  may  be  influenced  to  a  large  extent  by 
local  conditions  and  prejudices,  as  well  as  having  recorded  certain 
opinions  and  figures  in  the  past  from  which  it  may  be  difficult  to 
withdraw.  This  in  time  may  cause  a  certain  amount  of  inaccuracy 
to  creep  into  the  reports  of  the  mine  management,  rendering  their 
use  hazardous.  A  knowledge  of  the  character  and  ability  of  the 
management  will  assist  in  a  determination  of  the  reliability  of  the 
data. 

A  mine  may  attain  a  given  production  and  the  estimates  of  the 
ore  reserves  may  be  correct;  at  the- same  time  it  may  be  difficult  to 
continue  outputting  the  figures  attained  in  the  past,  because  the 
mining  work  has  not  been  properly  done.  It  may  be  found  that,  in 
order  to  attain  a  high  rate  of  production,  the  work  of  filling  the 
stopes  or  otherwise  supporting  the  working  places  has  been  neg- 
lected to  such  an  extent  that  some  of  the  largest  and  best  producing 
stopes  will  have  to  .be  completely  or  partly  shut  down  until  the 
heavy  ground  is  secured.  Not  only  is  the  output  affected  by  such 
a  state  of  affairs,  but  naturally  the  working  costs  will  have  a 
tendency  to  be  higher  for  at  least  a  temporary  period  of  time. 

Another  condition  that  is  sometimes  met  in  operating  mines  is 
that  the  management,  in  order  to  fulfill  promises  that  have  been 
made,  will  pick  the  eyes  out  of  the  mine,  in  other  words,  mine  an 
excessive  proportion  of  the  best  grade  of  ore  with  a  view,  of  course, 
of  maintaining  a  high  rate  of  production.  Many  orebodies  lend 
themselves  to  such  a  proceeding.  Shoots  may  exist  in  the  orebody 
which  have  a  higher  value  than  the  general  average  tenor  of  the 
remainder,  and  if  this  better  grade  ore  is  drawn  on,  in  too  great  a 
proportion,  the  time  must  eventually  come  when  it  will  be  ex- 
hausted and  the  remaining  ore  available  for  stoping  will  show  a 
lower  average  tenor.  In  studying  mine  plans  the  limits  of  these 
ore-shoots  are  not  always  definitely  located,  so  that  unless  great 
care  is  exercised  in  attempting  to  gauge  the  value  of  a  property  on 
second-hand  data  it  may  lead  to  an  improper  conclusion. 

Recently  I  had  occasion  to  check  up  a  report  by  two  well 
known  German  engineers,  and  the  difference  in  the  methods  em- 
ployed by  them  and  that  generally  accepted  as  good  practice  by 
American  and  English  engineers  is  quite  worth  discussing  for  the 
lessons  taught  thereby. 

Here  was  a  mine  in  which  they  estimated  ore  reserves  amount- 
ing to  more  than  150,000  tons.  This  figure  was  determined  by 
measuring  with  a  planimeter  the  area  stoped  as  shown  on  the  mine 
plan,  which  was  drawn  on  a  scale  of  1 :1000,  and  correlating  with 
it  the  recorded  output  of  the  mine.  In  this  way  a  figure  to  repre- 
sent the  tonnage  of  ore  per  square  metre  of  orebody  stoped  was 
obtained,  and  the  unstoped  ground  was  assumed  to  yield  a  similar 
tonnage,  of  course,  of  the  same  assay  value.  As  a  check  on  the 
smelter  returns,  about  20  samples  were  taken  in  various  parts  of 


WRITING  REPORTS  123 

the  mine.  The  deposit  was  a  fissure  vein,  averaging  a  little  over 
1  foot  in  width  (35  cm.),  and  there  were  no  vertical  development 
openings  on  the  orebody. 

I  do  not  think  that  good  American  or  English  practice  would 
sanction  the  estimation  of  tonnage  under  such  conditions.  The 
history  of  the  mine  showed  that  about  $450,000  had  been  spent  on 
it,  perhaps  a  great  proportion  of  it  uselessly,  in  excess  of  the 
actual  return,  and  that  during  the  latter  portion  of  the  period  it 
was  in  active  operation,  before  it  fell  into  the  hands  of  the  mort- 
gagee, the  eyes  were  picked  out  of  the  mine. 

In  making  this  estimate  of  ore  reserves,  the  engineers  were 
accepting  second-hand  data.  There  was  no  real  assurance  that 
the  surveys  were  properly  made  and  that  all  the  stoped  areas 
were  shown  on  the  plans.  It  will  readily  be  seen  that  on  omission 
of  any  stoped  ore  would  seriously  affect  the  tonnage  calculations 
and  show  a  greater  quantity  of  ore  per  square  metre  than  was 
actually  recovered. 

The  next  great  point  of  difference  was  in  the  manner  of  cal- 
culating the  average  assay  value  of  the  orebody.  Our  own  prac- 
tice demands,  as  has  been  explained  in  the  preceding  pages,  that 
samples  be  taken  at  regular  intervals,  with  an  accurate  determina- 
tion of  the  width  sampled.  In  the  case  under  discussion,  the  en- 
gineers took  the  smelter  returns  in  combination  with  the  mine 
records  of  tonnage  and  calculated  back  that  the  ore  averaged  a 
stated  amount,  after  allowing  for  losses  in  mining  and  concentration. 
The  records  of  the  mine  showed  that  they  had  shipped  arsenical  con- 
centrate and  copper  concentrate,  which  had  been  obtained  from  sev- 
eral different  veins.  A  cursory  inspection  of  the  underground  work- 
ings was  sufficient  to  show  that  the  principal  orebody  carried  much 
more  copper  than  other  parts  of  the  mine,  yet  the  same  average  value 
for  copper  was  given  to  this  ore  as  to  the  remainder.  The  German 
engineers'  figures  showed  an  average  value  of  3%  of  copper.  A 
number  of  check  samples  taken  by  me  demonstrated  an  average  value 
of  between  8  and  10%  of  copper  in  the  copper  vein. 

There  is  no  doubt  in  my  mind  that  this  orebody  could  be  mined 
and  treated  in  such  a  manner  as  to  yield  a  better  result  than  the 
averagfi  given  in  the  report.  The  mistake  made  and  one  in  which 
we  are  concerned  is  that  the  existing  methods  of  mining  and  con- 
centration were  accepted  as  efficient.  Even  assuming  that  the  mining 
methods  could  not  be  very  much  improved  upon,  considering  local 
conditions,  the  concentrator  left  a  great  deal  to  be  desired.  It  was  a 
most  inefficient  plant  and  the  tailing  probably  assayed  as  much  as  the 
original  ore.  Further,  this  ore  was  a  more  or  less  massive  sulphide 
and  a  very  considerable  percentage  could  have  been  hand-picked  with 
better  results  than  crushing  the  whole  lot  through  a  small  mesh  with 
heavy  losses  on  inefficient  tables.  The  acceptance  of  results  of  this 
kind  is  extremely  risky  and  may  lead  to  disaster. 

The  engineer  examining  a  mine  must  be  in  a  position  to  pass  on 
the  efficiency  of  the  methods  employed,  and  it  is  obvious  in  the  case 


124  MINE    SAMPLING   AND   VALUING 

cited,  that  acceptance  of  second-hand  information,  such  as  that  de- 
scribed, may  lead  into  difficulties  of  all  kinds.  Here  was  a  place 
where  improved  methods  could  easily  be  installed,  to  give  a  result 
entirely  different  from  that  shown. 

In  opposition  of  this,  another  case  comes  within  my  experience, 
which  also  refers  to  a  mine  on  the  continent  of  Europe,  which  was 
reported  on  by  an  engineer  sent  out  from  London  by  an  English  corn- 
pay.  It  was  a  copper  mine  which  had  been  in  successful  operation 
for  nearly  a  century,  with  very  much  the  same  type  of  ore  deposit  as 
that  described,  but  in  this  case,  generally  speaking,  the  proper  methods 
of  treatment  had  been  adopted.  Hand-picking  was  resorted  to,  to  as 
great  an  extent  as  possible,  in  order  to  sort  out  the  friable  and  valu- 
able copper  mineral  in  lumps,  thus  preventing  the  losses  inevitable 
once  it  was  crushed  and  submitted  to  water  concentration.  It  is  a 
well  known  fact  that  chalcopyrite,  when  finely  crushed,  has  a  great 
tendency  to  float  and  in  water  concentration  serious  losses  result. 

This  property  was  bought  by  the  English  company.  The  engineer 
discarded  the  old  method  of  treatment,  designed  a  concentrating  plant 
in  which  the  whole  of  the  ore  was  to  be  crushed  to  10-mesh  and  de- 
cided to  install  two  blast-furnace  plants  to  treat  the  concentrate  in- 
stead of  using  reverberatories  as  had  properly  been  used  all  the  many 
years  the  mine  was  in  operation.  Within  18  months  the  company 
had  exhausted  its  capital  and  lost  the  property. 


CHAPTER  XVI. 

SALTING. 

The  art  of  salting  is  probably  as  ancient  as  the  art  of  rnining 
itself.  In  the  early  days  of  mining  in  the  western  part  of  the 
United  States,  salting  probably  reached  its  highest  stage  of  de- 
velopment. As  the  present  time,  however,  it  is  not  attempted  so 
frequently,  probably  because  we  are  better  educated  now  and  mine 
valuing  is  more  scientific  than  it  formerly  was.  There  are  prob- 
ably 100  different  ways  of  salting  and  99  ways  of  finding  it  out, 
as  a  sag'e  friend  of  mine  very  tritely  puts  it.  Salting  may  be  de- 
fined as  the  act  of  fraudulently  increasing  the  value  of  a  sample 
of  ore  for  purposes  of  deception,  although  it  may  also  mean  any 
addition  of  valuable  mineral  to  the  sample  beyond  what  legiti- 
mately belongs  there,  so  that  we  may  say  a  person  is  salting  him- 
self. 

Of  the  two  general  classes  of  salting,  one  is  to  prepare  the 
exposures  beforehand  and  the  other  to  tamper  with  the  samples 
after  they  have  been  taken.  The  latter  is  the  more  customary 
method.  At  various  places  in  these  pages,  attention  has  been 
called  to  the  possibility  of  salting  and  the  necessary  precautions 
suggested.  At  no  time  during  the  entire  course  of  the  work  should 
the  engineer's  vigilance  be  relaxed,  not  even  after  the  sample 
is  taken.  It  sometimes  happens  that  great  care  is  used  during  the 
whole  of  the  underground  sampling  operations  and  caution  thrown 
to  the  wind  during  the  crushing  operations  or  the  assaying. 

In  the  early  days  of  mining,  before  mine  valuation  had  attained 
anything  like  the  scientific  level  it  now  occupies  and  when  valua- 
tion was  done  by  so-called  practical  men,  who  took  a  few 
samples  underground  in  a  perfunctory  way,  and  when  the  more 
usual  method  of  testing  a  property  was  by  taking  out  sufficient 
ore  to  make  a  mill  run,  all  sorts  of  roguery  was  indulged  in  and 
the  literature  of  mining  contains  many  illustrations  of  the  clever 
manner  in  which  the  examining  engineer  was  fooled,  or  attempts 
made  to  fool  him.  In  those  days  breaking  out  samples  by  means 
of  dynamite  was  not  taboo  as  at  the  present  time  and  a  favorite 
pastime  of  the  swindler  was  to  load  the  dynamite  cartridge  with 
gold  dust,  which  would,  of  course,  be  distributed  with  the  ore  on 
the  explosion  of  the  dynamite.  Another  favorite  method  was  to 
salt  the  ground  with  gold  dust  inserted  in  the  cracks  of  the  rock 
or  in  vugs  and  the  injection  of  gold  solution  in  the  samples  after 
they  had  been  taken.  During  mill  runs  it  was  an  easy  matter  to 
add  amalgam  to  the  ore  in  the  bins.  In  alluvial  washing  it  is 
easy  to  spit  tobacco  juice  containing  gold  dust  into  a  sample,  or 


126  MINE   SAMPLING   AND    VALUING 

for  a  man  to  conceal  gold  in  his  finger  nails,  which  he  liberated 
into  the  pan  containing  the  sample  for  washing. 

Mr.  Simon*  tells  of  an  attempt  to  salt  him  in  Siberia  in  the 
examination  of  an  alluvial  property.  Repeated  panning  tests 
failed  to  reveal  the  existence  of  gold  in  profitable  quantities,  but 
at  the  earnest  solicitation  of  the  owners  bulk  tests  were  made.  As 
the  gravel  had  to  be  carted  some  distance  to  water,  someone  al- 
ways accompanied  the  carts  from  the  excavation  to  the  washing 
place  and  great  care  taken  that  the  sample  was  not  interfered 
with  during  the  washing  operations,  and  notwithstanding  this, 
each  cart  load  revealed  pay  dirt.  The  fraud  was  not  discovered 
until  an  empty  cart  was  examined  before  filling,  as  no  one  up  to 
that  time  had  accompanied  it  on  its  return  journey.  This  in- 
spection revealed  a  harmless-looking  cigarette  paper  containing 
the  'salt,'  after  which  no  more  pay  results  were  obtained. 

An  attempt  was  once  made  to  salt  me  in  the  examination  of 
an  alluvial  property.  In  various  parts  of  the  mine  measured  quan- 
tities of  gravel  were  washed  without  revealing  pay  dirt.  The 
owner  was  an  engineer  of  good  reputation ;  hence  I  was  far  from 
suspecting  him.  He  insisted  that  there  must  be  some  accident, 
because  his  own  work  demonstrated  that  the  ground  developed 
contained  sufficient  gold  to  permit  of  it  being  worked  at  a  profit, 
and  he  asked  to  be  allowed  to  demonstrate  this  by  putting  several 
cubic  yards  through  the  washer.  No  objection  was  made  and  the 
gold  was  afterwards  collected  from  the  sluice  boxes  and  yielded 
almost  exactly  the  amount  claimed  by  him.  Unfortunately,  how- 
ever, among  the  gold  collected  from  the  sluice  boxes  was  a  nugget 
of  peculiar  shape  that  I  identified  as  having  seen  among  some 
specimen  gold,  which  he  had  previously  shown  me,  so  I  deter- 
mined that  I  would  stick  to  the  results  obtained  by  the  panning. 
Not  long  after  that  the  property  was  shut  down. 

The  /necessity  for  carefully  guarding  one's  samples,  after  they 
have  been  sacked,  can  be  illustrated  by  another  story  from  actual 
practice.  A  firm  in  New  York  was  asked  by  a  client  to  interest 
themselves  in  a  gold  mine  in  the  South.  The  engineer  sent  to 
make  the  inspection  returned  to  New  York  and,  on  assay,  his 
samples  showed  an  average  value  of  about  $10  per  ton.  His 
principals  were  convinced  from  a  long  experience  in  Southern  gold 
mining  that,  although  nothing  is  impossible,  at  the  same  time  it  was 
highly  improbable  that  any  such  orebody  existed  in  the  South.  A  re- 
examination  by  another  engineer  demonstrated  the  average  value 
of  the  quartz  to  be  less  than  $2  per  ton.  Upon  investigation  it 
turned  out  that  engineer  No.  1  had  been  presented  by  the  seller 
with  a  sample  of  unusual  occurrence  of  monazite  sand  containing 
gold.  The  sample  was  in  a  bottle  about  half  empty,  and  the  re- 
mainder had  been  used  to  salt  his  samples,  as  proved  by  panning. 
He  afterwards  remembered  having1  left  them  lying  at  one  part 
of  the  property  while  he  went  off  somewhere  else  to  take  another 

*Lecture,    Diggers    Club,    London,    1913. 


SALTING  127 

lot.  Don't  let  your  samples  out  of  your  sight  and  even  then  be  sure 
that  they  are  securely  tied  and  sealed  ivith  a  distinctive  seal  and 
locked  up  in  a  mail  sack  or  other  safe  receptacle. 

Perhaps  the  easiest  thing  to  salt  is  gold  ore,  but  even  samples 
of  copper  ore  have  been  salted  by  the  addition  of  one  or  two  small 
lumps  of  high-grade  ore.  In  a  sample  running  just  under  grade, 
the  addition  of  a  small  lump  of  60%  copper  ore  may  make  the 
difference  between  commercial  and  non-commercial  ore.  To  do 
this,  the  salter  must  get  at  the  engineer's  samples,  but  with  the 
safeguards  suggested  there  is  little  danger. 

Only  one  other  point  need  be  emphasized,  namely,  that  the 
examining  engineer  is  responsible  for  his  samples  up  to  the  very 
last  and  he  must  make  sure  that  the  preparation  of  the  final 
sample  for  assay  is  done  by  responsible  men.  It  would  indeed  be 
foolish  to  spend  weeks  in  making  a  mine  examination,  if  in  the 
end  the  samples  were  turned  over  to  an  irresponsible  person  for 
reduction  and  assay.  Local  assayers  interested  in  the  camp  are 
often  dishonest  and  the  'back-door'  assay,  or  the  'pencil'  assay,  is 
not  unknown. 

Sometimes  an  incompetent  man  will  falsify  his  returns.  As 
a  case  in  point,  some  years  ago  in  the  examination  of  a  tin  mine 
in  Tasmania  where  the  tin  occurred  in  a  body  of  massive  pyrite, 
I  sent  my  samples  to  an  assayer  of  good  reputation  in  Melbourne, 
and  the  average  of  the  results  he  returned  showed  the  ore  to  con- 
tain about  2^4%  tin.  At  the  same  time,  one-fifth  of  the  total 
number  of  samples  were  sent  to  another  assayer  to  be  checked, 
his  results,  arriving  a  few  days  later,  averaged  less  than  5/2%- 
As  the  mine  had  produced  considerable  specimen  tin  and  a  great 
deal  of  alluvial  tin  had  been  washed  from  the  creeks  in  the  vicinity 
of  the  property,  I  was  at  first  inclined  to  believe  that  the  second  as- 
sayer did  not  know  his  business,  although  he  likewise  was  a  man  of 
good  reputation  and  long  experience.  A  third  and  a  fourth  assayer 
were  called  in,  only  to  corroborate  the  low  results  obtained  by  No.  2. 
In  the  meantime  No.  1  was  interviewed  and  he  insisted  on  the  cor- 
rectness of  his  results. 

To  settle  the  business  it  was  deemed  advisable  to  bring  two  half- 
ton  lots  from  the  property,  which  were  crushed  and  quartered  down 
by  one  of  my  assistants,  and  the  resulting  samples  given  a  thorough 
test,  with  the  result  that  it  was  demonstrated  that  the  ore  was  not  of 
commercial  grade.  It  took  two  years  to  find  out  what  had  happened 
— assayer  No.  1  in  the  meantime  having  gone  out  of  business.  Then 
I  learned  that,  not  being  able  to  get  the  results  he  expected,  and 
wishing  to  please  me,  he  penciled  in  the  results,  his  excuse  being  that 
he  did  not  think  that  2%  or  3%  of  tin  made  any  difference.  This 
is  one  of  the  pitfalls  that  it  is  difficult  to  guard  against,  especially  in 
cases  where  an  engineer  is  at  a  distance  from  good  assayers  and 
must  place  himself  in  the  hands  of  outsiders.  However,  if  the  pre- 
caution is  taken  to  keep  duplicate  samples  of  the  pulps  and  have 


128  MINE    SAMPLING   AND    VALUING 

them   checked    elsewhere,   there   is    little   likelihood   of   the    engineer 
being  fooled. 

Sometimes  as  an  additional  safeguard,  a  few  lumps  of  clean  ore 
are  taken  and  in  the  assay  office  these  are  carefully  washed  and 
assayed  separately.  Any  serious  difference  between  the  results  of 
these  assays  and  the  regular  samples  is  sufficient  cause  for  suspicion 
and  investigation.  Another  subterfuge  sometimes  adopted  is  to  have 
a  few  bags  of  waste  rock  mixed  with  the  other  samples  and,  if  they 
are  all  salted  alike,  detection  is  inevitable. 


CHAPTER  XVII. 

PROSPECTS. 

The  vast  majority  of  engineers  are  called  upon  from  time  to  time 
to  examine  mines  that  are  only  partly  developed,  so  they  may  be  classed 
as  prospects.  Prospects  may  be  divided  into  two  classes:  (1)  New 
discoveries;  (2)  re-opened  old  mines.  I  do  not  recollect  ever  having 
seen  a  definition  of  a  prospect,  but  I  presume  the  generally  accepted 
meaning  among  the  profession  is,  that  a  prospect  is  a  mine  that  has 
no  ore  blocked  out  and  whose  value  in  consequence  is  entirely  pros- 
pective. If  we  cannot  calculate  the  existence  of  any  tonnage  of  ore, 
we  cannot  calculate  any  net  profit  and  the  property  can  only  have  a 
prospective  value.  The  question  then  arises,  what  price  can  the  en- 
gineer's client  afford  to  pay  for  the  prospect,  or  is  he  justified  in 
paying  any?  It  is  easy  enough  for  the  engineer  to  dodge  the  risk 
and  make  an  adverse  report,  but  in  doing  so  he  may  deprive  his 
client  of  a  large  possible  profit. 

New  Discoveries. — No  man  can  see  into  the  ground,  but  some 
men  can  make  a  better  guess  than  others,  by  -which  I  mean  to  say, 
as  I  have  already  indicated  elsewhere,  that  some  men  are  closer 
observers  than  others  and,  therefore,  can  better  read  the  indications 
that  exist  and  in  making  a  guess  are  more  likely  to  arrive  at  the  real 
position.  Many  a  prospect  is  worth  the  expenditure  of  the  money 
required  for  its  development  and,  if  the  conditions  warrant,  the  en- 
gineer is  justified  in  recommending  such  expenditure.  It  must  be 
understood  by  this,  of  course,  that  not  only  must  the  geological, 
mining,  and  economic  conditions  be  taken  into  consideration,  but  also 
the  financial  requirements.  In  general,  it  may  be  stated  that  when 
the  development  openings  on  a  prospect  are  in  ore  and  bottom  in  ore, 
further  expenditure  may  be  recommended,  if  the  other  conditions  are 
satisfactory. 

Personally  I  do  not  believe  that  one  is  ever  justified  in  recom- 
mending the  expenditure  of  money  where  the  development  open- 
ings are  in  ore  below  commercial  grade,  except  in  the  case  of  a 
copper  property.  In  the  latter  class,  as  is  well  known,  due  to 
surface  oxidation,  the  upper  portions  of  the  orebody  may  be 
leached  of  their  copper  contents  and,  until  the  zone  of  secondary 
enrichment  is  reached,  commercial  grade  ore  cannot  be  expected. 

Re-opened  Old  .Mines. — In  the  re-opening  of  old  mines  we 
are  often  confronted  with  false  evidence.  The  old  workings  do 
not  always  tell  the  truth,  nor  are  the  deductions  that  one  may 
reasonably  make  from  the  evidence  they  present  to  be  depended 
upon  except  with  great  reservation.  In  examining1  the  old  work- 
ings of  a  mine  we  are  inclined  to  take  the  width  of  the  old  stopes 
as  prima  facie  evidence  as  to  the  width  of  the  ore  mined  and, 


130  MINE    SAMPLING    AND    VALUING 

further,  we  are  likely  to  start  with  the  hypothesis  that  the  attain- 
ments of  modern  times  and  the  great  improvements  in  mining 
and  metallurgical  methods  will  enable  us  to  handle  material  that 
the  old-timers  could  not  possibly  have  treated  at  a  profit.  No 
greater  fallacy  ever  existed. 

Barring  a  few  specific  innovations,  such  as  the  cyanide  process, 
the  chief  improvements  that  have  been  made  within  the  last  fifty 
years  have  been  on  the  mechanical  side,  so  that  we  are  now  able, 
nay  compelled,  to  treat  larger  tonnages  in  order  to  obtain  a 
profit.  The  old-timers  were  good  miners  and  good  metallurgists, 
but  they  only  treated  small  quantities  of  material.  Their  con- 
centration methods  were  good  and  their  smelting  was  good.  The 
great  change  that  has  been  brought  about  is  largely  due  to  eco- 
nomic conditions.  The  purchasing  power  of  gold  has  decreased 
so  that  comparatively  the  metals  are  not  worth  as  much  as  they 
formerly  were.  As  an  instance  we  may  cite  the  unsuccessful  ef- 
forts of  modern  companies  working  the  gold  mines  in  Egypt. 
Labor  is  now  much  more  costly  than  formerly  and,  in  the  days 
of  low  output  and  cheap  labor,  hand-sorting  and  other  means  of 
securing  a  high  extraction  was  prevalent  and  cheap,  so  that  the 
size  of  stopes  is  not  necessarily  an  indication  that  values  have 
been  continuous  for  the  entire  width  opened  up. 

Even  at  the  present  day  in  remote  places  we  can  find  a  sur- 
vival of  old-time  conditions.  A  few  years  ago  I  visited  a  mine  in 
Austria,  which  has  been  operating  continuously  for  a  century,  and 
whose  early  history  dates  back  to  the  Celts.  A  walk  through  the 
stopes  would  lead  one  to  believe  that  the  orebody  had  been  10 
to  15  ft.  wide,  but,  as  a  matter  of  fact,  the  stope  was  carried  that 
width  because  of  the  existence  of  two  streaks  of  high-grade  ore, 
separated  by  material  practically,  if  not  quite,  barren.  The  only 
possible  index  to  this  condition  was  the  gob  which  had  been 
thrown  back  into  the  mine  as  filling1. 

In  examining  an  old  mine,  if  all  the  old  stopes  are  caved  and 
inaccessible  for  inspection,  it  is  quite  easy  to  be  deceived,  and 
unless  there  is  positive  evidence  at  hand,  the  engineer  is  not 
justified  in  placing  any  great  weight  on  the  evidence  afforded  by 
the  appearance  of  caved  workings.  I  know  of  two  copper  mines 
that  were  re-opened  and  the  old  workings  were  extensive  in  each 
case.  In  one  of  them,  assays  of  the  old  pillars  showed  ore  running 
as  high  as  35%  copper  and  yet  development  below  the  water  level 
demonstrated  the  non-existence  of  ore  in  commercial  quantities. 
The  magnitude  of  the  orebody,  as  indicated  by  the  old  stopes,  was 
entirely  falsified  by  the  subsequent  development. 

Another  point  with  old  mines  is  the  existence  of  dumps  of 
waste  rock.  All  through  the  states  of  North  Carolina,  South 
Carolina,  and  Georgia  in  the  United  States,  are  old  mines  on  which 
in  some  cases  are  enormous  dumps  of  white  quartz — a  very 
suspicious  circumstance.  Many  of  these  old  mines  have  been  re- 
opened, capital  having  been  largely  influenced  by  former  State 


PROSPECTS  131 

Geologists'  reports,  and,  speaking  generally,  it  may  be  said  that 
not  in  onq  instance  has  any  profit  resulted  therefrom. 

The  existence  of  old  dumps  is  unquestionably  evidence  that 
the  ore  occurred  either  as  narrow  streaks,  as  isolated  bunches  or  as  a 
stockwork,  and  that  excessive  amounts  of  waste  rock  had  to  be 
mined  with  it.  As  a  matter  of  fact,  the  history  of  these  Southern 
mines  indicates  that  certainly  in  their  later  years  they  were  not 
operated  at  a  profit,  but  were  run  solely  in  the  interest  of  a  Stock 
Exchange  clique,  who  manipulated  the  shares  up  and  down  on 
the  discovery  or  exhaustion  of  any  particular  pocket. 

It  is  much  better  to  meet  with  the  entire  absence  of  waste 
dumps  on  an  old  mine,  because  this  indicates  that  everything 
taken  out  was  treated,  and  while  this  is  not  an  absolute  assurance 
that  the  property  was  at  all  times  operated  at  a  profit,  yet  it 
indicates  an  orebody  of  workable  dimensions.  A  narrow  orebody 
or  one  occurring  in  bunches,  pockets,  or  similar  form,  will  neces- 
sitate the  extraction  of  some  country  rock  with  the  ore. 


CHAPTER   XVIIL 

A  FEW  SPECIAL  CASES. 

Miners  as  Samplers. — Sometimes  an  engineer  reaches  a  mine 
with  one  or  more  assistants  to  find  there  is  a  great  deal  more 
work  to  be  done  than  had  been  originally  contemplated  and,  as 
the  examination  must  necessarily  be  completed  within  a  given 
period,  he  is  compelled  to  seek  outside  assistance.  It  is  usually 
practically  impossible  to  get  any  technically  trained  assistants 
near  the  property,  who  can  be  relied  upon  and  the  only  other 
alternative  is  for  the  engineer  to  make  use  of  ordinary  miners. 
This  is,  of  course,  open  to  the  serious  objection  that  such  men  are 
not  trained  samplers  and  may  be  of  questionable  loyalty;  however, 
the  average  miner,  through  his  familiarity  with  the  tools,  can  in 
a  short  time  be  taught  how  to  take  a  fairly  accurate  sample  and, 
while  the  results  obtained  cannot  be  given  the  same  dependence 
as  the  work  of  the  regular  staff,  yet  by  close  supervision  suf- 
ficiently reliable  results  may  be  secured.  The  scratch  lot  should 
not  be  allowed  to  work  alone  but  in  company  with  one  of  the 
more  trusted  men,  taking  alternate  samples,  so  that  the  work  can 
be  watched.  After  the  completion  of  the  sampling,  the  engineer 
or  his  assistants  must  take  a  number, of  check-samples  as  a  check 
on  the  work  done  by  the  miners.  Any  factor  of  safety  that  is 
necessary  can  be  applied  and  in  this  way  the  likelihood  of  serious 
error  is  avoided.  Frankly,  this  procedure  is  open  to  many  ob- 
jections, but  under  the  conditions  described  there  seems  to  be  no 
other  alternative,  and  means  the  difference  between  getting  a  com- 
plete or  even  incomplete  sampling  of  the  mine.  A  part  sampling 
means  incomplete  data  and,  therefore,  may  be  the  cause  of  an 
incorrect  opinion,  whereas,  using  the  miners  under  proper  super- 
vision and  check-sampling  afterwards  the  satisfactory  completion 
of  the  work  in  hand. 

Checking  Surveys. — In  mine  examinations,  it  is  quite  usual,  on 
account  of  the  difficulty  of  checking,  to  accept  certain  information 
as  reliable.  Particularly  is  this  the  case  with  reference  to  the 
depth  of  shafts  or  other  data  of  a  like  character.  Very  often, 
especially  with  mines  that  have  been  in  existence  for  a  number 
of  years,  mistakes  have  been  made  in  the  surveys  and  the  error 
has  been  carried  through  by  successive  surveyors.  The  distance 
between  levels  should  always  be  checked,  as  the  tonnage  cal- 
culations are  based  on  such  measurements,  and  if  second-hand 
data  is  accepted  serious  error  may  arise.  I  know  of  just  such  a 
case  where  a  shaft  was  about  100  ft.  shallower  than  it  was  g'en- 
erally  believed  to  be.  Many  possible  errors  in  surveying  will 
naturally  occur  to  experienced  engineers  and  it  is  needless  to  mul- 


A   FEW    SPECIAL   CASES  133 

tiply  the  illustrations  here,  sufficient  to  point  out  the  necessity 
of  checking  all  surveys  on  which  estimates  are  based.  In  any 
case  it  must  be  apparent  that  many  beautifully  drawn  plans  have 
been  prepared  by  incompetent  surveyors  and  the  engineer  must 
take  a  sufficient  number  of  important  measurements  to  feel  con- 
fident that  the  figures  he  has  taken  into  his  calculations  are  reliable. 

Sometimes  trouble  arises  from  the  fact  that  the  examining 
engineer  fails  to  make  sure  that  the  property  he  is  examining  is 
the  one  his  clients  have  under  option.  Before  much  time  is  spent 
on  the  mine  it  should  be  the  duty  of  the  engineer  to  go  round 
the  boundaries  of  the  mining  claims  and  satisfy  himself  that  the 
main  openings  are  actually  within  the  claims  in  which  they  are 
supposed  to  be.  As  a  further  guide,  a  combined  surface  and 
underground  plan  should  be  prepared  for  the  purpose  of  actually 
showing  the  position  of  the  underground  workings  with  reference  to 
the  surface  boundaries.  As  an  illustration  of  the  usefulness  of  this, 
I  may  quote  an  experience  of  my  own  that  resulted  beneficially  to 
my  clients,  and  that  of  another  engineer  whose  neglect  caused  serious 
loss  to  his  clients. 

In  the  examination  of  the  'L'  mine,  I  found  that  in  portions 
of  the  property  the  vein  would  dip  out  of  the  side  lines  at  about 
300  ft.  below  the  existing  level,  it  was,  therefore,  necessary  to  peg 
out  a  number  of  other  claims  to  protect  the  deep  ground.  For- 
tunately we  were  able  to  do  this,  as  the  ground  had  not  been 
previously  located.  It  can  easily  be  seen  that  a  case  such  as  this 
might  arise,  where  the  ground  desired  was  held  under  different  own- 
ership and  unless  satisfactory  arrangements  could  be  made  the  pros- 
pective value  of  the  property  would  be  definitely  limited.  I  know  of 
just  such  a  case  where  the  examining  engineer  did  not  take  the 
precaution  to  secure  the  deep  ground,  or  to  warn  his  client  regarding 
the  necessity  of  it,  with  the  result  that  after  the  purchase  was  com- 
pleted the  new  owner  discovered  that  the  land  covering  the  dip  of 
the  lode  had  been  secured  by  a  third  person,  and  this  eventually 
caused  disaster. 

Dressing  Mines  for  Sale. — Vanity  is  just  as  apt  to  lead  a  mining 
engineer  astray  as  any  other  man.  The  average  expert  is  very  prone 
to  regard  with  contempt  the  ordinary  rule-of-thumb  operator.  Often- 
times such  a  man  is  not  given  credit  for  being  as  astute  as  he  really 
is,  with  the  result  that  the  engineer  fails  to  judge  the  position  cor- 
rectly. An  actual  case  of  this  sort  is  that  of  the  'B'  mine  which  was 
floated  a  few  years  ago  under  particularly  brilliant  auspices,  and  had 
among  its  shareholders  a  number  of  well  known  mining  engineers. 
The  purchasers  were  deceived  in  the  value  of  the  property  more  by 
appearances  than  anything  else.  The  mine  unquestionably  was 
dressed  for  sale,  although  I  do  not  think  that  any  misrepresentations 
were  made  by  the  owner,  he  giving  the  experts  a  free  hand  to  carry 
out  the  inspection  in  any  way  they  desired.  Everywhere  about  the 
place,  both  surface  and  underground,  things  were  in  such  a  condition 


134  MINE    SAMPLING   AND   VALUING 

as  to  give  an  idea  of  bad  management  and  slovenliness,  but  the  owner 
in  opening  his  mine  sent  to  the  mill  only  the  easily-treated  ore,  leaving 
behind  the  refractory  material  ostensibly  because  his  plant  was  not 
capable  of  treating  low-grade  material.  It  is  true  that  the  plant  was 
not  capable  of  treating  the  ore.  The  examining  engineer  believed 
that  by  introducing  up-to-date  methods  of  treatment,  installing  a 
first-class  mill,  putting  the  mine  in  proper  order,  supplying  it  with 
adequate  hoisting  machinery  and  other  plant  and  in  general  intro- 
ducing improved  metallurgical  and  mining  methods,  he  would  be  able 
to  reduce  the  costs  to  such  ah  extent  as  to  be  able  to  treat  the  ore 
at  a  profit. 

The  purchase  of  the  mine  was  accordingly  completed  and  after 
a  great  deal  of  money  had  been  spent  in  equipment,  the  operations 
Droved  to  be  a  failure.  The  engineer  made  a  mistake.  The  mistake 
is  one  that  many  men  are  prone  to  make,  namely,  they  backed  their 
ability  to  make  a  profit  by  improving  the  methods  of  others.  Some- 
times all  the  factors  are  not  taken  into  consideration,  factors  which 
are  difficult  to  ascertain,  largely  because  they  are  not  so  much 
scientific  as  technical.  In  the  particular  case  under  discussion,  in 
checking  working  costs  under  the  owner's  management,  the  engineer 
overlooked  the  fact  that  the  owner  was  able  to  operate  more  econom- 
ically than  a  company,  because  he  received  no  salary  and  on  account 
of  the  small  scale  of  his  operations,  he  was  able  to  fill  several  positions 
at  once.  Besides  he  was  an  extremely  capable  man,  better  than  the 
average  mine  manager,  with  boundless  energy,  working  for  his  own 
profit,  and  by  practicing  every  economy,  he  was  able  to  attain  a  work- 
ing cost  that  could  not  be  done  under  any  other  condition. 

This  is  only  one  illustration  of  a  number  of  a  similar  kind  that 
are  common  enough  in  practice  and  are  apt  to  fool  any  but  the  most 
experienced  engineer,  because  it  is  difficult  to  bring  ourselves  to  be- 
lieve that  actual  figures,  as  shown  by  the  books  of  a  going  mine, 
are  not  a  true  measure  of  the  operations.  T.  A.  Rickard1  relates  an 
experience  somewhat  of  this  character  in  connection  with  his  exam- 
ination of  the  Camp  Bird  mine,  a  number  of  years  ago.  His  esti- 
mates of  the  tonnage  and  value  of  the  ore  reserves  have  been  fully 
borne  out  by  subsequent  operations,  but  the  working  costs  have  been 
greatly  exceeded,  as  he  says  because  he  neglected  to  take  into  account 
the  inevitable  higher  expense  connected  with  the  operation  of  a 
mining  property  by  a  London  company.  It  is  only  in  rare  cases  that 
one  is  justified  in  assuming  the  ability  to  reduce  working  costs  below 
those  actually  attained  at  the  time  of  the  examination. 

Misrepresentation  of  Facts. — There  are  more  fools  than  knaves 
in  the  mining  business,  but  the  engineer  must  not  allow  himself  to 
be  a  victim  of  either  class  if  he  wishes  to  make  a  success  of  his  pro- 
fession. An  engineer  should  gather  his  own  data  and  not  accept 
statements  made  by  interested  parties.  He  must  guard  not  only 
against  wilful  misrepresentation,  a  practice  not  altogether  unknown 

1Mining    and    Scientific    Press,    May    24,    1913. 


A   FEW    SPECIAL   CASES  135 

to  sellers  of  mines,  but  also  against  misrepresentation  due  to  ignor- 
ance. Even  an  honest  man  in  a  spirit  of  loyalty  to  his  employer 
will  put  the  best  possible  face  on  a  business  and  may  allow  an  exam- 
ining engineer  to  be  misled  by  the  mere  fact  of  his  silence.  There 
may  be  a  question  whether  an  engineer  representing  a  seller,  is  com- 
mitting a  breach  of  professional  ethics  by  withholding  salient  facts 
which  the  visiting  engineer  may  have  difficulty  in  ascertaining.  An 
engineer  must  remember  that  a  man  selling  a  horse  only  talks  of 
the  horse's  good  points  and  leaves  the  buyer  to  find  out  the  bad  ones. 

Some  years  ago  I  examined  a  prospect  in  the  United  States,  and 
the  manager  in  showing  me  over  the  ground  pointed  out  what  he 
considered  the  persistence  of  the  ore  through  the  three  principal 
openings,  which  were  not  in  a  continuous  line,  but  were  at  divergent 
angles  to  one  another.  There  was  very  little  work  done,  so  I  had 
considerable  difficulty  in  sizing  up  the  position.  The  manager  was 
convinced  in  his  own  mind  that  these  openings  were  all  on  the  same 
deposit,  but  I  could  not  make  my  observations  fit  in  with  any  such 
theory.  One  of  these  three  openings  was  an  adit  driven  at  the  foot 
of  the  hill  and  had  entered  a  few  feet  of  ore.  This  gave  the 
clue  to  the  position,  for  at  this  cross-cut  I  was  able  to  determine 
what  I  considered  the  strike  of  the  lode  and  subsequent  development 
work  carried  out  on  this  hypothesis  opened  up  a  large  tonnage  of  ore. 

I  know  of  a  mine  in  Norway  which  was  owned  by  two  engineers, 
one  of  whom  accompanied  an  examining  engineer,  representing  a 
would-be  purchaser.  The  length  of  the  orebody  was  represented  as 
its  width.  It  is  needless  to  say  that  a  very  brief  examination  satisfied 
the  visitor  as  to  this  fact,  and  an  unfavorable  report  was  the  result. 

A  case  of  wilful  misrepresentation  happened  a  few  years  ago  in 
Australia,  and  the  victim  was  a  capable  engineer.  He  was  probably 
in  a  hurry  and  consequently  did  not  take  the  trouble  to  crawl  into 
and  examine  all  the  old  workings,  nor  did  he  take  the  trouble  to 
thoroughly  inspect  the  ore  exposures  to  which  the  most  importance 
was  attached.  The  old  workings  in  the  mine,  where  the  principal 
work  had  been  done,  were  opened  by  an  inclined  shaft  from  which 
three  levels  at  short  intervals  had  been  driven  a  short  distance  along 
the  strike  of  the  lode.  There  were  one  or  two  other  shafts  from  the 
surface  which  it  was  evident  were  in  barren  ground  as  they  were 
stated  to  be  off  the  lode. 

A  new  vertical  shaft  had  been  sunk  on  the  hanging  side  of  the 
lode  and  at  a  depth  of  200  ft.  a  short  cross-cut  struck  the  northern  end 
of  an  orebody.  The  drift  to  the  south  was  in  ore,  but  the  drift  to  the 
north  was  in  a  black,  slick,  apparently  highly  crushed  material  due  to 
movement.  The  cross-cut  itself  was  continued  through  a  body  of 
quartz,  which  was  represented  to  be  a  cross-course  which  had  caused 
the  crushing  of  the  schist  in  the  north  drift,  but  it  was  really  the 
foot-wall  of  the  lode,  the  hanging  side  being  schist.  A  subsequent 
examination  demonstrated  these  points,  as  well  as  the  fact  that  from 
the  old  workings  a  connection  had  been  made  with  one  of  the  other 
old  .shafts,  which 'the  first  engineer  did  not  observe.  Had  he  done 


136  MINE    SAMPLING   AND   VALUING 

so,  he  would  have  found  that  the  connection  was  driven  on  the  strike 
of  the  lode,  but  showed  no  ore,  and  that  from  the  other  shaft  two 
cross-cuts  were  driven,  one  east  and  one  west,  without  showing 
any  ore.  The  second  engineer  arrived  at  the  conclusion  that 
the  ore  occurred  as  a  cigar-shaped  lens,  the  top  of  which  was 
in  the  workings  from  the  inclined  shaft  and  the  bottom  of  which 
was  just  touched  by  the  cross-cut  on  the  200-ft.  level  from  the  main 
vertical  shaft,  therefore,  there  was  no  likelihood  of  opening  up  any 
large  tonnage  of  ore.  In  other  words,  the  mine  was  bottomed. 
Subsequent  work  by  others  demonstrated  the  correctness  of  these 
views. 

The  manager  for  the  owners  at  the  time  of  these  examinations 
was  interested  in  the  sale  of  the  property  and,  being  unscrupulous, 
he  did  not  hesitate  to  try  and  hoodwink  the  purchasers'  engineer. 
The  moral  of  this  story  is  that,  regardless  of  the  inconveniences  it 
may  cause,  every  working  in  the  proximity  of  an  orebody  should  be 
inspected  unless  the  examining  engineer  is  willing  to  assume  a 
negative  value  for  such  portions  of  the  workings  as  are  not  visited. 
If  an  orebody  has  been  opened  up  for  a  considerable  length  there 
may  be  some  justification  in  assuming  its  continuance  for  some  ad- 
ditional distance ;  where,  however,  only  a  small  amount  of  develop- 
ment work  has  been  done  and  there  are  other  openings  on  the  strike 
of  the  lode,  these  must  be  inspected  for  the  information  that  may  be 
obtained  from  them.  If  the  first  engineer  had  only  taken  the  pre- 
caution to  have  some  rubbish  cleared  away  from  in  front  of  the  con- 
nection above  mentioned,  he  would  have  saved  his  clients  considerable 
money  and  himself  some  loss  of  reputation. 


CHAPTER  XIX. 

SPECIFIC  GRAVITY. 

Before  any  tonnage  calculations  can  be  undertaken,  it  is  necessary 
to  determine  the  specific  gravity  of  the  ore  so  as  to  arrive  at  the  number 
of  cubic  feet  per  ton  of  ore  in  place.  The  accompanying  table  shows 
the  specific  gravity  of  various  ores.  The  figures  given  are  averages 
taken  from  Dana's  Mineralogy  and  will  be  found  close  enough  for 
ordinary  practice.  From  the  surveys  and  sampling  measurements, 
the  cubic  contents  of  any  block  of  ground  may  be  ascertained  and  to 
get  at  the  actual  number  of  tons  of  ore  in  the  block,  the  number  of 
cubic  feet  per  ton  of  ore  in  place  must  be  determined.  This  is  de- 
pendent on  two  things:  (1)  the  specific  gravity  of  the  ore;  (2)  its 
compactness.  There  are  several  methods  of  determining  specific 
gravity,  which  are  set  forth  below. 

The  open  spaces  in  the  orebody,  such  as  cracks  and  vugs,  affect  the 
actual  weight  of  material  that  can  be  mined  from  a  given  volume  of 
ground,  so  that  after  determining  the  specific  gravity  of  the  ore  itself, 
an  allowance  must  be  made  for  these  open  spaces. 

In  addition  to  the  ordinary  methods  of  determining  specific  gravity 
by  laboratory  methods,  occasionally  it  may  be  desirable  for  the  sake  of 
greater  accuracy,  to  take  out  a  given  volume  of  ore  and  weigh  it. 

A  suitable  spot  is  selected  in  the  orebody  and  a  measured  block  of 
any  given  dimension  of  1  to  3  or  more  cubic  feet  bulk  is  cut  out  and 
squared  to  dimensions  by  moil  or  chisel  and  the  debris  thus  secured 
carefully  collected  and  weighed.  A  simple  calculation  will  then  give 
the  number  of  cubic  feet  of  ore  in  place  to  the  ton. 

The  following  is  taken  from  Furman's  'Manual  of  Assaying': 

"The  specific  gravity  of  any  body  is  the  weight  of  that  body  as  com- 
pared with  the  weight  of  an  equal  volume  of  another  body  which  is 
assumed  as  a  standard.  The  standard  taken  for  solids  and  liquids  is  dis- 
tilled water;  for  gases  and  vapors,  dry  air  and  occasionally  hydrogen. 
All  determinations  of  solids  and  liquids  must  be  made  at  the  same  tem- 
perature. The  temperature  usually  adopted  is  60°  Fahrenheit." 

Of  the  various  methods  of  ascertaining  specific  gravities  of  sub- 
stances, the  two  here  quoted  are  the  only  ones  that  need  be  taken  into 
consideration  in  mine  examination  work : 

1.     Lumps. 

Weigh  first  in  the  air,  suspending  the  lump  of  ore  from  the  beam  of 
the  balance  by  a  piece  of  horse-hair,  and  then  in  distilled  water  whose 
temperature  is  60°  F.  Let  W  =  the  weight  in  air,  W  =  the  weight 
in  water,  and  Sp.gr.  =  the  specific  gravity ;  then 

W 
Sp.gr.  .  —    -^ 


138 


MUTE    SAMPLING   AND   VALUING 


2.     Fragments  or  powder. 

Fill  a  specific-gravity  bottle*  with  distilled  water  whose  temperature 
is  60°  F.,  and  weigh  it.  This  weight  =W.  Weigh  the  substance  in 
the  air.  This  weight  =  W.  Now  introduce  the  weighed  substance  into 
the  flask,  fill  it  with  distilled  water,  and  weigh.  This  weight  —  W" : 


Sp.gr.  = 


W 


(W+W)    — 


SPECIFIC  GRAVITY  OF  MINERALS.1 


Metal 

Form 

Mineral 

Average 
Specific 
Gravity 

Lb.  Wt. 
per 
Cu.  Ft. 

No.  of  Cu.  Ft. 
per  ton 
2000  Ib. 

Antimony  

Native 

6    7 

417   8 

4   8 

Arsenic  . 

Sulphide 

Native 

Stibnite 
Orpiment 

4.6 

5  8 

286.8 
361  6 

7.0 

5  5 

Barium  
Bitumen  

Sulphide 
Sulphate 
Carbonate 

Realgar 
Barite 
Witherite 
Carbon 

3.5 
4.5 
4.3 
1  5 

218.2 
280.5 
268.1 
93  5 

9.2 
7.1 
7.4 
21  4 

Calcium  
Coal  

Carbonate 

Sulphate 
Fluorite 
Phosphate 
Anthracite 

Calcite 
Aragonite 
Gypsum 
Fluorspar 
Apatite 

2.7 
3.0 
2.3 
3.2 
3.2 
1  5 

168.4 
187.1  ' 
143.4 
199.4 
199.4 
93  5 

11.9 
10.7 
13.9 
10.0 
10.0 
21  4 

Bituminous 

1  3 

81  0 

24  6 

Cobalt 

Lignite 
Sulphide 

Linnaeite 

4  9  * 

305  5 

6  5 

Copper  

Co-Ni-Arsenide 
Co.As.S. 
Arsenate 
Native 

Smaltite 
Cobaltite 
Erythrite 

6.8 
6.2 
3.0 
8.9 

424.0 
386.6 
187.1 
554  9 

4.7 
5.2 
10.7 
3.6 

Sulphide 
Cu-Fe-S 
Cu-Fe-S 
Cu-S-As 
Cu-S-Sb 
Oxichloride 
Oxide 

Chalcocite 
Chalcopyrite 
Bornite 
Enargite 
Tetrahedrite 
Atacamite 
Cuprite 
Melaconite 

5.7 
4.2 
5.0 
4.4 
4.9 
3.8 
6.0 

355.4 
262.0 
311.8 
274.4 
305.5 
236.9 
374.1 

5.6 
7.6 
6.4 
7.3 
6.5 
8.4 
5.3 

Gold  

Sulphate 
Carbonate 

Silicate 

Native 

Chalcanthite 
Malachite 
Azurite 
Chrysocolla 
Dioptase 

2.2 
3.9 
3.7 
2.2 
3.3 
19.0 

137.2 
243.1 
230.7 
137.2 
205.7 
1184.7 

14.6 
8.2 
8.7 
14.6 
9.1 
.1.7 

Iron  

Sulphide 

Arseno-sulphide 
Oxide 
Titanic  Oxide 
Oxide 

Carbonate 

Pyrite 
Marcasite 
Pyrrhotite 
Arsenopytite 
Hematite 
Menaccanite 
Magnetite 
Limonite 
Siderite 

5.1 
4.8 
4.6 
6.0 

5.0 

4.8 
5.0 
3.8 
3.8 

318.0 
299.3 
286.8 
374.1 
311.8 
299.3 
311.8 
236.9 
236.9 

6.3 
6.7 
7.0 
5.3 
6.4 
6.7 
6.4 
8.4 
8.4 

*If  a  specific-gravity  bottle  is  not  at  hand,  take  a  thin  glass  flask  with  a  narrow 
neck  and  scratch  a  mark  on  the  neck.  The  flask  is  to  be  filled  to  this  mark  in  the 
determinations. 

JAverage  figures  are  used,  based  on  those  in  Dana's  Mineralogy. 


SPECIFIC  GRAVITY 


139 


SPECIFIC  GRAVITY  OF  MINERALS— Cont. 


Metal 

Form 

Mineral 

Average 
Specific 
Gravity 

Lb.  Wt. 
per 
Cu.  Ft. 

No.  of  Cu.  Ft. 
per  ton 
2000  Ib. 

Lead 

1.  Sulphide 

Galena 

7   3 

455    1 

4  4 

Manganese.  .  .  . 

Mercury  

Molybdenum.  . 
Nickel  

Platinum  

2.  Carbonate 
3.  Sulphate 
4.  Chromate 
5.  Phosphate 
Dioxide 

Carbonate 
Silicate 
Native 
Sulphide 

Arsenical 

Native 

Cerussite 
Anglesite 
Crocoite 
Pyromorphite 
Pyrolusite 
Psilomelane 
Wad 
Rhodochrosite 
Rhodonite 

Cinnabar 
Molybdenite 
Millerite 
Niccolite 

6.5 
6.4 
6.0 
7.0 
4.8 
4.2 
3.5 
3.6 
3.6 
14.4 
9.0 
4.6 
5.6 
7.5 
17.5 

405.3 
399.0 
374.1 
436.4 
299.3 
262.0 
218.2 
224.4 
224.4 
897.9 
561.1 
286.7 
349.2 
467.6 
1091.2 

4.9 
5.0 
5.3 
4.6 
6.7 
7.6 
9.1 
8.9 
8.9 
2.2 
3.5 
7.0 
5.7 
4.3 
1.8 

Silver  

Native 

2.8 

174.6 

11.5 

Sulphur 

Sulphide 
Telluride 
Ag-Au-Te 
Ag-Au-Te 
Ag-Sb-S. 
Ag-As-S. 
Ag-As-S. 
Chloride 
Native 

Argentite 
Hessite 
Petzite 
Sylvanite 
Pyrargyrite 
Stephanite 
Proustite 
Cerargyrite 

7.3 
8.5 
9.0 
8.0 
5.8 
6.3 
5.5 
5.4 
2  1 

455.1 
530.0 
561.1 
498.8 
361.6 
392.8 
342.9 
336.7 
130  9 

4.4 
3.8 
3.6 
4.0 
5.5 
5.1 
5.8 
6.0 
15  3 

Tin 

Oxide 

Cassiterite 

6.8 

424  0 

4.7 

Tungsten  
Zinc  

•  Rocks 

Quartz 

Sulphide 
Fe-Mn-W 
Ca-W. 
Sulphide 
Oxide 
Sulphate 
Carbonate 
Silicate 

Stannite 
Wolframite 
Scheelite 
Blende 
Zincite 
Goslarite 
Smithsonite 
Calamine 

Composition 
Silica 

4.5 

7.3 
6.0 
4.0 

5.7 
2.0 
4.4 
3.7 

2  6 

280.5 
455.1 
374.2 
249.4 
355.4 
124.7 
214.4 
230.7 

162  1 

7.1 
4.4 
5.3 
8.0 
5.6 
16.0 
7.3 
8.7 

12  3 

Andesite  

2  9 

181  0 

11  1 

Basalt 

Diabase 

3  0 

187  0 

10.6 

Diorite 

Granite 

2.7 

168.0 

11.9 

Limestone 

2.7 

168.0 

11.9 

Porphyry  
Rhyolite  

C'-S- 

2.73 
2.4 

170.0 
149.6 

11.8 
13.4 

Sandstone  .... 
Schist 

1 

2  7 

168  0 

11  9 

Shale 

2  6 

162  1 

12  3 

CHAPTER  XX. 

THE  SAMPLING  OF  PLACER  DEPOSITS 

By  Chester  Wells  Purington. 

Preliminary. — In  the  following'  chapter,  which  relates  to  the 
sampling  of  alluvial  deposits  only,  I  am  obliged  to  make  the  barest 
reference  to  the  great  subject  of  the  cost  of  placer  mining.  The 
costs  of  dredging,  of  hydraulicking,  of  hydraulic  elevating,  and  of 
drift  or  channel  mining,  are  of  an  intricate  nature,  and  their  analysis 
necessitates  a  treatment  of  many  component  elements.  No  placer 
engineer  is  qualified  to  make  an  examination  of  a  gravel  deposit  who 
has  not  a  thorough  knowledge  of  the  cost  of  gravel  excavation  and 
alluvial  metal  saving,  as  exemplified  in  the  principal  placer  districts 
of  the  world.  Volumes  have  been  written  on  the  cost  of  gold  dredg- 
ing in  various  countries,  and  yet  in  every  new  case,  the  situation 
and  environment  of  the  particular  property  which  the  engineer  has 
to  examine  will  control  the  cost.  To  know  the  gross  recoverable 
value  per  cubic  yard  of  a  given  mass  of  gravel,  be  it  ever  so  rich, 
is  of  no  use  unless  the  engineer  has  in  his  possession  that  knowledge 
of  previous  operations  under  similar  conditions  which  will  allow 
him  to  form  a  conservative  estimate  of  the  cost  of  extracting  and 
marketing  the  contained  metal  and  getting  the  money  returns  for 
the  same. 

Placer  operations,  which  deal  as  a  rule  with  much  larger  quan- 
tities of  material  than  do  lode  operations,  frequently  necessitate  a 
proportionately  heavier  expenditure  for  preparation  and  equipment. 
This  expenditure  as  well  as  purchase  price  must  be  entirely  redeemed 
in  the  ordinarily  short  life  of  the  placer  mine. 

The  engineer  should  not  fail  to  allow  for  every  possible  item  of 
working  and  redemption  cost  and  should  not,  for  the  sake  of  false 
economy,  base  his  estimate  for  equipment  on  the  use  of  cheap  and 
futile  machinery.  Many  a  spectacular  hydraulic  mine  running  today 
is  a  costly  luxury  to  the  owners  and,  notwithstanding  the  numerous 
successes  attained,  many  a  gold  dredge  lies  stranded  in  the  grassy 
valleys  of.  California  or  Colorado,  and  on  the  wind-swept  tundras 
of  Alaska. 

For  the  sake  of  uniformity  all  references  to  gold  values  are  made 
in  terms  of  U.  S.  currency,  which  is  standard  in  placer  sampling, 
And  it  should  be  further  said  that  all  weighings  of  placer  gold  and 
platinum  samples  are  done  in  milligrams.  Placer  gold  ranges  from 
650  to  950  fine  and  varies  with  each  locality.  One  gram  of  pure 
gold  is  worth  $0.6646.  A  safe  value  for  all  ordinary  cases  is  to  count 
20  milligrams  to  one  cent,  or  1  mg.  as  worth  0.05  of  1  cent. 


THE   SAMPLING   OF   PLACER   DEPOSITS  141 

The  only  instance  in  which  the  chemical  assay  is  allowable  in 
connection  with  placer  sampling  is  in  the  case  of  alluvial  tin  deposits 
where  a  heavy  concentrate  is  obtained  in  the  pan,  consisting  of  fine 
cassiterite,  magnetite,  wolframite,  garnet,  rutile,  etc.,  impossible  to 
separate  mechanically  in  the  field.  As  this  concentrate  represents 
the  commercial  product  obtainable  in  the  sluice-box  or  on  the  dredge 
table,  the  sample  must  be  submitted  to  chemical  assay  before  the 
value  of  the  concentrated  product,  which  it  is  mechanically  possible 
to  save,  can  be  arrived  at. 

It  is,  however,  an  extraordinary  fact  that  reports  on  alluvial  gold 
properties  are  occasionally  submitted  with  lists  of  samples  which  have 
been  submitted  to  fire  assay.  Any  engineer  who  would  fire-assay 
the  samples  from  a  gold  gravel  mine  might  be  retained  to  make  one 
report.  He  would  probably  never  have  the  opportunity  to  make 
another  one. 

General. — The  procedure  in  sampling  gravel  deposits  in  the  ma- 
jority of  cases  with  which  the  engineer  has  to  deal,  approaches  much 
more  nearly  to  mathematical  exactitude  than  does  the  sampling  of 
deposits  in  situ.  However  extensively  an  original  mass  of  ore  in 
place,  be  it  a  vein,  ore  bed,  or  impregnation,  may  be  developed,  and 
however  thoroughly  the  examining  engineer  applies  those  laborious 
methods  of  determining  the  value  which  have  been  so  ably  described 
in  the  foregoing  pages,  there  is  always  in  such  cases  an  unknown 
factor  which  concerns  the  extension  of  the  ore-mass  in  length  or 
depth  beyond  the  exposed  faces.  This  unknown  factor,  in  the  case 
of  gravel  deposits  either  is  non-existent  or  it  exists  in  a  very  sub- 
ordinate degree.  Broadly  speaking,  the  value  of  a  placer  deposit 
containing  a  workable  quantity  of  an  economic  mineral  depends  on  its 
area  or  horizontal  extension,  while  the  value  of  an  ore  deposit  depends 
on  its  extension  in  depth  (vertical  or  inclined). 

Placer  deposits  which  are  too  deep  to  be  economically  drilled 
or  tested  by  prospecting  shafts,  are  seldom  of  importance,  except  in 
the  case  of  deep-lying  channels  of  gravel  covered  by  ancient  lavas. 
Just  so  far  as  gravel  or  placer  sampling  calls  for  the  exact  methods 
of  the  mathematician,  or  civil  engineer,  in  the  same  measure  it 
cannot  be  said  to  allow  room  for  the  exercise  of  that  fine  intellectual, 
and  nearly  indescribable  quality  called  judgment,  to  which  atten- 
tion has  been  so  emphatically  called  in  the  preceding  pages, 
as  an  essential  qualification  of  what  may  be  described  as  the 
hard-rock  mining  engineer.  While  it  is  gratifying  to  the  placer  en- 
gineer to  know  in  advance  that  he  will  in  every  case  deal  with  pre- 
determined knowable  quantities,  so  that  there  is  no  room  for  a  guess, 
he  knows,  too,  that  his  work  is  far  less  interesting;  that  it  is  on  a 
lower  intellectual  plane  and  approaches  more  nearly  to  the  rule-of- 
thumb  methods  of  the  craftsman,  than  does  the  intricate  duty  of  the 
engineer  engaged  in  sampling  a  large  deposit  in  situ,  which  last 
entails  a  fundamental  knowledge  of  the  great  subject  of  ore  deposits, 
keen  judgment,  wide  metallurgical  training,  and  in  addition  an 


142  MINE    SAMPLING   AND   VALUING 

ability  to  apply  the  precise  methods  essential  in  all  engineering  prac- 
tice. 

Because  the  bulk  of  valuable  metal  won  from  alluvial  deposits  is 
gold,  and  because  an  increasing  quantity  of  gold  is  won  every  year 
by  the  process  of  dredging,  the  present-day  placer  engineer,  in  nine 
cases  out  of  ten,  is  called  on  to  sample  gold  gravel  deposits  suitable, 
or  assumed  to  be  suitable,  for  gold  dredging. 

The  reduction  of  the  methods  of  sampling  gold-dredging  areas 
to  a  scientific  basis  is  a  development  of  the  last  fifteen  years,  and  is 
thus  a  much  more  recent  acquisition  to  engineering  practice  than  the 
sampling  of  underground  ore  workings.  There  have  come  to  be 
recognized  certain  well  defined  methods,  one  of  which  the  engineer 
must  choose,  if  possible,  in  accordance  with  the  character  and  situa- 
tion of  the  gravel  deposit  in  question.  He  must  also  recognize  that 
certain  attendant  conditions,  such  as  the  nature  of  the  bedrock,  the 
presence  of  boulders,  and  excessive  amount  of  clay,  too  great  or  too 
shallow  depth  of  ground,  insufficient  water  supply,  or  other  disad- 
vantages entirely  apart  from  the  gold  contents  of  the  gravel,  may 
justify  an  unfavorable  opinion  of. the  property,  irrespective  of  results 
obtained  from  the  pit-sinking  or  drilling  returns.  These  special 
accessory  conditions  are  as  important  as  the  gold  tenor  to  the  success 
of  a  dredging  enterprise. 

Classes  of  Deposits. — Before  describing  the  sampling  methods,  a 
brief  description  and  classification  of  gravel  deposits  will  be  given, 
as,  notwithstanding  the  present  and  increasing  interest  in  the  gold- 
dredging  industry,  important  and  highly  productive  deposits  of  gravel 
exist  which  must  be  sampled  when  opportunity  arises,  with  a  view 
to  exploitation  by  entirely  different  methods. 

The  term  'gravel  deposit,'  as  used  in  this  chapter,  includes  only 
those  detrital  deposits  which  may  be  exploited  for  their  valuable 
metallic  contents  and  are  referred  to  as  placers  or  alluvial  deposits. 
This  naturally  excludes  all  deposits  of  gravel  which  are  worked 
merely  for  use  as  road  metal,  railway  ballast,  in  concrete  work  and 
the  like.  Placers  include,  according  to  a  decision  of  the  United 
States  Supreme  Court,  all  deposits  in  which  a  valuable  mineral  occurs 
in  the  softer  material  which  covers  the  earth's  surface,  as  distinguished 
from  occurrences  in  the  rocks  beneath. 

As  the  mineral  sought  sometimes  lies  in  a  matrix  of  fine  sand, 
or  even  entirely  in  the  crevices  of  the  bedrock  on  which  the  alluvial 
deposit  rests,  the  term,  'gravel  deposit'  is  obviously  not  always  ap- 
plicable. Likewise,  in  the  case  of  alluvial  deposits,  or  those  in  which 
the  metalliferous  layer  is  a  result  of  concentration  from  the  rotting 
of  rock  and  veins  in  place  to  a  greater  or  less  depth,  the  material 
cannot  be  called  gravel  in  the  strict  sense  of  the  word ;  therefore,  it 
is  preferable  to  use  the  simple  word  'placer'  to  describe  all  valuable 
alluvial  deposits,  which  may  be  exploited  by  the  various  forms  of 
surface  mining.  The'  only  kind  of  alluvial  deposit  which  cannot  be 
properly  called  a  placer  is  that  which  lies  too  deep  to  be  profitably 


THE   SAMPLING   OF   PLACER   DEPOSITS  143 

exploited  by  surface  cuts,  as,  for  example,  the  Tertiary  basalt- 
covered  gravel  channels  of  California  and  Australia  or  the  deep 
channels  of  Fairbanks,  Alaska,  or  Bodaibo,  Siberia,  covered  with 
frozen  silt  over-burden  and,  for  the  sake  of  simplicity,  these  will  be 
described  by  the  term  'buried  placers.' 

The  valuable  metals  sought  in  placer  deposits  are  in  the  order 
of  their  importance — gold,  tin,  platinum,  monazite,  and  the  metals 
of  the  osmiridium  group.  Placers  containing  diamonds,  rubies,  and 
other  precious  stones,  are  of  such  rare  occurrence  that  they  need 
not  be  considered  here. 

Gravel  deposits  may  be  classified  as  follows : 

1.  Buried  placers — Deposits  buried  under   from  75   to   1,000  ft. 
of  overlying  alluvial  or  lava. 

2.  Creek   placers — Placers    in,    adjacent   to,    and   at   the   level   of 
small   streams. 

3.  Hillside   placers — Placers     on     slopes,     intermediate    between 
creek  and  bench  claims. 

4.  Bench    placers — Placers    in    ancient    stream    deposits    from    50 
to  300  ft.  above  present  streams. 

5.  River  bar  placers — Placers  on   gravel  flats  in  or  adjacent  to 
the  beds  of  large  streams. 

6.  Gravel   plain    (tundra)    placers — Placers   in   the   coastal   plain 
of  Seward  peninsula. 

7.  Seabeach  placers — Placers  adjacent  to  seashore  to  which  the 
waves  have  access. 

8.  Lake  bed  placers — Placers  accumulated  in  the  beds  of  present 
or  ancient  lakes ;  generally  formed  by  landslides  or  glacial  damming. 

These  eight  classes  of  deposits  will  be  considered  in  sequence, 
from  the  standpoint  of  their  respective  adaptability  to  different 
methods  of  taking  the  samples. 

The  method  of  washing  the  material  obtained  in  the  sample  will 
be  considered  later. 

Class  i.  Buried  Placers. — These  must  be  worked  in  almost  every 
case  by  underground  or  drift  mining,  and,  as  the  engineer  is  so 
rarely  called  on  to  sample  such  a  deposit,  the  subject  may  be  dis- 
missed with  a  few  words.  Should  the  deposit  be  covered  by  100 
feet  or  more  of  the  over-burden,  whether  it  be  basalt  or  silt  and 
overlying  barren  gravel,  six-inch  drills,  operated  by  steam  or  gaso- 
line power,  must  be  employed  in  the  vast  majority  of  cases.  Rarely 
a  so-called  channel  or  drift  mine  is  for  sale  in  California,  where 
working  faces  on  the  valuable  bed  of  gravel  are  available  for  sam- 
pling. Such,  for  example,  was  the  examination  of  the  Red  Point 
mine  in  Placer  county,  California,  several  years  ago. 

Certain  drift  mines  in  Australia  and  in  the  Lena  and  Nerchinsk 
districts  of  Siberia,  where  a  heavy  barren  alluvial  covers  the  pay 
gravel  from  100  to  250  ft.  thick,  have  been  worked  for  long  periods 
and  may,  when  offered  for  sale,  present  certain  blocks  of  gravel  in 
the  form  of  thin  horizontal  sheets,  opened  by  tunnels  on  three  or 


144  MINE    SAMPLING   AND   VALUING 

four  sides.  A  thorough  sampling  of  the  Industrial  Company's 
property  in  the  Lena  district  of  Siberia  was  made  in  1897-98  by  two 
American  engineers,  samples  being  broken  from  the  faces  exposed 
underground,  in  much  the  same  manner  as  would  be  done  in  the 
case  of  a  horizontal  bed  of  lead  or  zinc  ore  in  limestone. 

Class  2,  Creek  Placers. — It  is  seldom  that  an  engineer  is  called 
on  to  sample  a  small  creek  deposit.  In  doing  so  he  will  bear  in  mind 
that  the  gold  will  probably  be  coarse  and  irregularly  distributed  and 
that  much  water  will  be  present,  in  all  probability  preventing  sampling 
by  pits.  General  conditions  will  favor  close  sampling  by  four-inch 
hand  drills,  as  topography  is  likely  to  be  steep,  and  as  workable  creek 
deposits  occur  only  in  new  and  uncivilized  countries.  Roads  will  be 
few  and  transport  of  heavy  power  drills  difficult. 

Under  the  class  of  creek  deposits,  comes  the  subordinate  class 
of  deposits  formed  by  decomposition  of  rock  in  place,  only  slightly 
concentrated  by  stream  action.  It  is  known  in  advance  by  the  exam- 
ining engineer  that  values  in  such  a  deposit  are  extremely  irregular 
and  minute  care  is  necessary  in  sampling.  Examples  of  this  type 
of  alluvial  deposit  are  found  in  Nigeria,  where  the  metal  sought  is 
tin,  and  in  Eastern  Russia,  where  such  deposits  have  been  worked 
for  gold  for  nearly  a  century.  The  well  known  'saprolite'  deposits 
which  were  at  one  time  highly  productive  in  Georgia  and  North 
Carolina,  U.  S.,  were  largely  of  this  type. 

Class  j.  Hillside  Placers. — These  occasionally  form  the  lateral 
extensions  of  creek  or  river  bar  placers,  the  whole  together  forming 
a  sufficiently  extensive  mass  of  gravel  to  make  exploitation  by  gold 
dredging  or  by  underground  drift  mining,  a  possibility. 

The  characteristic  of  such  deposits  is  an  approximately  level  cross- 
section  of  the  bed  rock  extending  from  wall  to  wall  of  the  valley. 
The  over-burden  gets  progressively  deeper  approaching  the  sides. 
Deep  drilling  with  power  drills  is  the  only  satisfactory  way  to  sample 
such  a  deposit. 

Class  4.  Bench  Placers — Include  those  rare  but  eagerly  sought 
gold  alluvials  which  are  suitable,  providing  water  supply  conditions 
are  right,  for  hydraulic  mining.  The  distinguishing  characteristic 
is  the  level  of  the  sill  of  rock  on  which  the  gravel  rests  relatively 
to  the  level  of  the  water  of  the  existing  stream,  contiguous  to.  which 
the  deposit  lies.  The  difference  between  these  levels  constitutes  the 
'dump  room/  in  other  words,  the  vertical  height  available  for  the 
tailing  from  the  hydraulic  sluices.  The  classic  occurrence  of  bench 
placers  is  the  Tertiary  river  system  of  California,  which  has  been 
elevated  and  dissected  by  present  streams  so  that  at  times  the  canons 
of  the  existing  streams  lie  hundreds  of  feet  below  the  rock  sills  or 
shelves  on  which  the  ancient  gravel  beds  were  deposited. 

The  thickness  of  ancient  benches  from  top  to  rock  floor  varies 
between  wide  limits.  Perhaps  the  commonest  thickness  which  the 
engineer  is  called  on  to  sample  is  from  30  to  100  ft.  Sampling  is 


THE   SAMPLING   OF   PLACER  DEPOSITS  145 

difficult  and   expensive,   and  the   choice   of  the   method   of   sampling 
depends  on  many  conditions. 

Pit-sinking  is  generally  impossible  on  account  of  depth.  If  the 
bench  is  intersected  longitudinally  by  a  present  stream,  so  as  to  ex- 
pose it  in  section,  side  creeks  may  be  taken  advantage  of  to  ground- 
sluice  a  certain  amount  of  the  gravel,  thus  giving  at  intervals  a 
representative  cut  from  top  to  bottom,  from  which  a  sample  of  50  to 
100  cu.  yd.  may  be  washed.  I  have  in  mind  a  gravel  bench  300  ft. 
thick,  and  over  2  miles  long,  in  Alaska,  which  was  sampled  in  that 
manner.  Drilling  with  hand  drills  may  be  practicable.  If  the  gravel 
bank  is  less  than  50  ft.  in  depth,  and  contains  no  large  boulders, 
this  method  is  preferable.  The  use  of  power  drills  may  be  imperative, 
and  in  this  case,  preparation  may  be  made  and  expense  allowed  for 
drilling  a  series  of  holes,  from  100  to  as  much  as  400  ft.  in  depth, 
the  cost  of  which  may  be  from  $500  to  $1,000  each. 

It  should  be  said  that  the  examination  of  large  hydraulic  prop- 
erties by  drilling  is  rarely  called  for,  as  such  mines  have  generally 
been  started  by  individual  miners,  and  a  working  option  is  given  to 
intending  purchasers,  who  are  content  to  operate  the  property  on  a 
gradually  increasing  scale  until  it  is  either  self-supporting  or  aban- 
doned as  too  poor  to  pay.  A  set  of  conditions  fully  as  important 
to  the  success  of  hydraulicking  bench  deposits  as  the  gold  content 
of  the  gravel,  relate  to  water  supply,  grade  of  streams,  and  perhaps 
most  important  of  all,  dump  room. 

Classes  5  and  6.  River  Bar  and  Gravel  Plain  Deposits. — These 
are  not  divided  by  any  sharp  line.  Generally  they  lie  adjacent  to 
creeks  and  small  rivers  and  in  them  the  bulk  of  the  world's  alluvial 
gold  is  distributed,  although  rarely  of  sufficient  richness  to  pay  to 
mine.  Deposits  of  this  character  present  the  best  physical  conditions 
for  dredging,  but  it  is  rare  indeed  that  the  gravel  is  valuable  enough 
to  pay.  They  are  generally  of  great  lateral  extent,  sometimes  several 
miles  wide,  and  vary  in  depth  from  15  to  75  ft.  and  'even  more.  They 
may  consist  of  gravel  from  surface  to  bedrock  or  may  be  covered 
with  an  over-burden  of  soil  or  peat. 

Dredging  is  the  only  method  by  which  they  are  exploitable,  and 
sampling  in  most  cases  must  be  done  by  drilling,  although  pit-sinking 
is  preferable  wherever  possible.  Conditions  in  these  deposits  are 
generally  eminently  suitable  for  drilling  with  6-in.  power  traction 
drills  of  the  Keystone  type.  The  ground  is  approximately  flat,  with 
a  hard  surface  on  which  the  drill  can  move  itself  easily  from  place 
to  place. 

Class  J.  Seabcach  Placers. — This  class  is  so  limited  that  it 
scarcely  merits  consideration.  In  only  a  few  places  in  the  world 
have  such  deposits  been  known  to  pay  the  expense  of  mining,  the 
most  notable  being  the  present  beach  at  Nome,  Alaska.  They  are 
the  resort  of  individual  miners,  who  operate  generally  with  simple 
rocker  or  sluice  box. 


146  MINE    SAMPLING   AND   VALUING 

Class  8.  Lake  Bed  and  Harbor  Deposits. — This  v.ery  limited  class 
of  alluvial  deposits  is  rarely  payable.  Gold  in  lake  beds  is  known 
and  recently  tin-bearing  alluvial  has  been  profitably  exploited  in 
harbors  in  Siam  and  similar  deposits  are  known  in  Cornwall.  To 
sample  such  a  deposit,  either  a  hand  or  power  drill,  mounted  on  a 
scow  or  barge,  should  be  used.  The  fine  character  of  the  alluvial 
detritus  generally  will  render  the  drilling  comparatively  expeditious 
and  inexpensive. 

Characteristics  of  the  Pay  Layer. — The  gold  or  other  valuable 
constituent  is  generally  associated  with  valueless  magnetic  sand  in  the 
bottom  layer  next  to  bedrock,  and  sometimes  entirely  in  the  bedrock 
crevices.  In  typical  so-called  gravel  wash,  the  pay  gravel  is  ordinarily 
recognized  by  certain  local  characteristics,  or  indications,  such  as  a 
blue  or  yellow  color,  the  presence  of  binding  clay,  relative  coarseness 
of  material,  or  the  like.  While  black  or  magnetic  sand  is  sometimes 
entirely  absent,  as  a  rule  the  pay  streak  contains  a  far  greater  amount 
of  oxide  of  iron  than  any  overlying  layer.  This  is  frequently  accom- 
panied or  even  entirely  replaced  by  garnet  sand,  the  so-called  'ruby 
sand'  of  the  miner.  When  there  are  igneous  rocks  in  the  vicinity 
other  heavy  materials,  such  as  spinel,  zircon,  or  rutile,  are  also  found 
with  the  gold. 

Origin  of  Auriferous  Plains. — A  short  description  of  the  origin 
of  those  gravel  deposits  which  present  physical  conditions  favorable 
to  dredging  follows.  In  the  formation  of  alluvials  the  wearing  down 
of  mountains,  consisting  of  rocks  penetrated  by  auriferous  veins, 
causes  the  formation  of  valleys  of  varying  width,  depth  and  length. 

When  these  valleys  are  of  small  length  but  great  width,  they  in- 
dicate that  stream  action  has  been  rapid,  and  the  decomposition  and 
distribution  of  the  detritus  have  gone  on  with  proportionate  rapidity. 
These  are  favorable  valleys  for  the  concentration  of  alluvial  gold.  On 
the  other  hand,  valleys  extremely  long,  narrow  and  of  comparatively 
shallow  width,  are  an  indication  that  erosion  has  been  slow  and  streams 
sluggish.  The  gold  will  travel  a  comparatively  short  distance  from 
its  source  and  will  be  distributed  in  narrow  and  short  pay  channels. 
Consequently  such  regions  do  not  give  promise  of  economic  alluvial 
conditions. 

The  controlling  factor  is  the  amount  of  elevation  to  which  the 
land  was  originally  subjected.  Comparatively  intense  and  protracted 
elevation  has  occurred  in  California,  and  far  north  to  the  Yukon,  re- 
sulting in  such  benches  as  occur  at  Georgia  Hill,  in  the  Forest  Hill 
divjde,  and  in  the  White  Channel  of  the  Klondike.  On  the  other 
hand,  in  regions  presenting  the  topographical  development  of  the 
northern  Altai  in  Siberia,  the  comparatively  narrow  valleys  retain 
the  same  width  for  great  lengths,  and  are  evidently  the  result  of  pro- 
cesses of  extremely  gentle  uplift  followed  by  necessarily  slow  de- 
gradation. 

The  fundamental  geographic  dissimilarity  between  these  two  con- 
ditions and  the  processes  resulting  therefrom  have  a  considerable  in- 


THE   SAMPLING   OF   PLACER   DEPOSITS  147 

fiuence  in  determining  whether  a  given  mass  of  auriferous  detritus 
will  prove  payable  or  unpayable.  In  other  words,  granting  that  the 
amount  of  gold  originally  contained  in  equal  masses  of  eroded  rock 
was  equal,  the  manner  of  its  subsequent  distribution  will,  in  the  case 
of  the  short,  wide  and  rapidly  eroded  valley,  be  a  factor  in  favor  of 
a  profitable  deposit,  while  in  the  case  of  the  long,  comparatively 
narrow  and  slowly  carved  valley  the  physiographic  causes  militate 
against  the  presence  of  an  extensive  sheet  of  profitable  gravel. 

Distribution  of  Pay  Channels  in  the  Plains. — Whether  or  not 
the  topographic  conditions  have  been  favorable  to  the  occurrence  of 
an  extensive  deposit,  it  is  almost  an  invariable  rule  that  the  most 
abundant  and  coarsest  gold  will  occur  in  defined  channels  as  a  result 
of  stream  deposition.  Considering  a  gravel  plain  deposit,  as  the 
type  most  frequently  to  be  examined,  the  course  of  the  present  stream 
which  meanders  through  such  a  plain  sometimes  affords  little  index 
of  the  actual  course,  width,  or  situation  of  the  valuable  gold-bearing 
channel  which  lies  beneath. 

Fine  colors  or  particles  of  gold  may  or  may  not  occur  in  the 
surface  alluvial,  but  this  gold  is  merely  the  result  of  annual  distribution 
and  re-distribution  by  floods  and  currents  which  disturb  the  upper 
layers  of  fine  gravel,  but  do  not  touch  the  bedrock  pay.  A  preliminary 
line  of  pits  or  drill  holes  directly  across  the  valley  will  quickly  dis- 
cover if  any  pay-channel  exists  below  the  flood  plain  and  also  determine 
its  situation  and  width.  The  definition  of  what  constitutes  pay  gravel 
naturally  varies  in  each  separate  case,  according  as  its  recoverable 
tenor  in  gold,  or  other  metal  does  or  does  not  leave  the  required 
margin  of  profit  above  the  estimated  cost  of  production.  The  entire 
gravel  plain  of  a  valley,  for  say  a  mile  in  width  and  five  miles  in 
length,  and  a  thickness  of  thirty  feet,  may  contain  gold,  but  the  pay- 
channel  width  and  length  in  this  plain  may  be  of  such  insignificant 
proportions  that  the  property  has  no  commercial  value. 

Preliminary  Remarks  to  Sampling.-— -The  placer  engineer  gener- 
ally has  an  indication  in  advance  as  to  what  character  of  deposit  he 
is  expected  to  sample.  If  it  is  to  be  a  dredging  property,  it  is  un- 
likely to  have  been  previously  worked.  He  should  be  on  his  guard, 
however,  both  in  the  case  of  dredging  ground,  and  of  hydraulic 
benches,  to  note  how  much  of  the  bottom  pay  gravel  has  been  drifted. 
The  term  'drifting'  in  alluvial  mining  is  applied  to  a  system  of  under- 
ground gravel  mining,  similar  to  the  long  wall  system  used  in  a  coal 
mine. 

The  placer  deposit  which  is  for  sale  is  rarely  one  where  actual 
work  is  in  progress,  or  has  been  done  for  the  purpose  of  develop- 
ment. Sampling  of  in  situ  deposits  can  be  done  only  after  a  con- 
siderable amount  of  development  work  has  been  accomplished  previ- 
ous to  the  visit  of  the  engineer.  The  gravel  sampler,  on  the  other 
hand,  has  to  block  out  the  reserves  by  the  very  process  of  sampling 
which  he  undertakes,  so  that  it  is  nothing  more  or  less  than  pros- 
pecting, or  dealing  with  quantities  which  are  unknown. 


148  MINE    SAMPLING    AND   VALUING 

As  a  corollary  to  this,  and  this  point  is  extremely  important,  it  should 
be  remembered  that  when  a  gravel  property  has  been  completely  sam- 
pled, it  has  also  been  completely  developed.  In  other  words,  as  one  is 
dealing  with  a  horizontal  instead  of  a  vertical  or  nearly  vertical  sheet, 
the  gravel  examination  places  the  whole  life  of  the  mine  in  sight, 
because  its  limits  can  be  actually  determined,  while  the  examination 
of  in  situ  deposits  can  only  refer  to  that  portion  of  the  life  of  the  mine, 
represented  by  the  ore  opened  up  by  the  existing  workings.  In  other 
words,  with  the  placer  deposit  there  is  no  prospective  ore.  These  two 
operations  interlock  or  overlap  in  the  case  of  disseminated  copper  de- 
posits and  horizontally  bedded  deposits  where  the  number  of  beds  is 
definitely  known. 

Conditions  Affecting  Value  of  Property. — As  in  the  case  of  ore- 
bodies  in  situ,  the  engineer  must  immediately  investigate  the  local  con- 
ditions controlling  the  cost  of  working.  Defective  titles,  difficulties  of 
transport,  inferior  quality  or  insufficient  supply  of  labor,  insufficient 
water  supply,  shortness  of  season  due  to  climate,  or  any  one  of  a  num- 
ber of  adverse  conditions  may  warrant  condemning  the  property  with- 
out any  drilling  whatsoever.  In  such  a  case,  the  engineer  saves  money 
for  his  client  by  immediately  leaving  the  property,  notwithstanding 
the  fact  that  a  heavy  expense  may  have  been  incurred  in  preparations. 

Instruments  and  Equipment. — Besides  the  necessary  special 
equipment  of  tools  and  implements,  the  following  essential  implements 
should  be  provided  in  any  placer  examination : 

Aneroid  barometer. 

Brunton  compass. 

Level  and  rods. 

Drawing  implements,  paper,  and  note  books. 

Carpenter's  pencils.' 

1  steel  tape. 

1  linen  tape. 

Blocks  of  drill  and  time  logs,  good  for  both  shafts  and  drill  holes. 

1  two-foot  rule. 

100  small,  strong,  1  by  fy&  in.  bottles  with  corks. 

500  sheets  light,  strong  bond  paper. 

1  powerful  pocket  lens. 

1  balance  sensitive  to  j/2  mg.  and  weights  with  no  glass  part. 
6  gold  pans  of  steel,  not  (thin)  iron. 

3  Ib.  quicksilver. 

Annealing  cups. 

Nitric  acid. 

Alcohol  lamp  and  alcohol. 

Magnet. 

Concentrate  dishes. 

Gummed  labels. 

2  dozen  6x8-in.  canvas  sacks. 
1   sheet  canvas. 

1  prospector's  pick. 


THE    SAMPLING   OF    PLACER   DEPOSITS 


149 


1  short-handled  shovel. 

3  galvanized  iron  panning  tubs  3  ft.  in  diameter  by  18  in.  deep. 
1   retort  stand,  alcohol,  and  stove  for  same. 
Panning  tubs  of  wood  or  galvanized  iron. 

If  several  panners  are  to  work  under  one  chief  engineer,  many  of 
the  above  will  be  duplicated. 

Rocker  and  Clean-up  Boxes. — A  most  essential  implement  is 
the  California  rocker,  which  is  shown  in  Fig.  22.  It  may  be  made 
smaller  than  this  in  proportion,  but  I  have  found  this  size  to  an- 
swer for  all  general  purposes.  Working  dimensions  for  construct- 
ing this  rocker  are  given  by  W.  H.  Radford  in  the  Mining  & 
Scientific  Press  of  June  8,  1912.  This  may  be  made  on  the  ground 
if  carpenters  are  available;  otherwise  it  must  be  made  beforehand 
and  transported  to  the  ground  in  knocked-down  condition.  A  dip- 
per (which  may  be  made  out  of  a  2-quart  tomato  can)  with  a 
handle  is  also  necessary. 


End  Election 


Fig   22.     CALIFORNIA   ROCKER. 


If  the  hand  drill  is  to  be  used,  a  special  clean-up  box  must  be 
made  about  12x18x12  in.  deep,  standing  on  legs  about  1  foot 
above  the  ground,  open  at  one  end,  and  fitted  with  a  gate,  into 
which  the  contents  of  the  sand-pump  is  dumped  and  under  which 
the  pan  is  placed.  If  the  power  drill  is  used,  large  and  strong 
clean-up  boxes  must  be  made  for  catching  the  pulp  drawn  up  with 
each  pumping  of  the  sand-pump. 

Hand  Pumps. — If  shaft-sinking  is  to  be  resorted  to,  it  will  be 
well  to  provide  two  or  more  ordinary  hand  pumps  of  the  bilge  or 
diaphragm  type,  with  3l/>  in.  suction,  and  3  ten-foot  lengths  of 
heavy  rope-wound  hose  and  couplings  for  each  pump,  together 
with  good  foot-valves  and  strainers  for  each.  Sometimes  the  re- 
sults of  an  examination  are  lost  for  the  lack  of  such  simple  equip- 
ment. 

Prospecting  by  Shaft  Sinking. — I  will  dismiss  with  a  few  words 
the  subject  of  prospecting  by  shafts,  as  this  desirable  method  is 


150  MINE    SAMPLING   AND   VALUING 

rarely  possible.  The  ground  is  laid  out  by  survey,  and  shafts  are 
sunk  at  intervals  of  100  to  200  ft.  in  lines  across  the  valley  or 
gravel  bench.  The  shafts  should  preferably  be  4^ft.  square,  and 
as  little  timber  used  as  possible.  A  cut  in  each  of  the  four  sides 
of  the  shaft  is  made  as  each  foot  in  depth  is  attained,  and  this 
gravel  is  hoisted  and  piled  separately.  The  rocker  is  set  up,  and 
the  panner  will,  after  determining  the  weight  and  cubic  contents  of 
a  bucket  of  gravel  used  for  measuring,  put  the  representative 
sample  from  each  foot  or  two  feet  through  the  rocker,  and  clean 
up.  The  number  of  colors  and  sizes  for  each  unit  of  depth  will  be 
entered  in  the  shaft  log,  and  then  the  gold  placed  in  the  concen- 
trate cup  or  bowl.  At  the  end  a  few  drops  of  quicksilver  will  col- 
lect all  the  gold  from  the  test,  and  the  amalgam  is  placed  in  a 
bottle  to  be  parted  in  the  evening  with  nitric  acid  and  the  gold 
weighed.  A  factor  on  the  safe  side  is  the  fact  that  the  recovered 
gold  weighed  after  parting  is  nearly  pure,  but  is  still  valued  as 
placer  gold.  When  colors  are  coarse,  amalgamation  is  not  necessary, 
as  the  cleaning  may  be  done  with  a  magnet  and  the  non-magnetic 
residue  blown  away. 

Shaft  sinking  varies  in  cost  from  $1  to  as  high  as  $5  per  foot. 
The  price  and  quality  of  labor,  amount  of  timbering  and  pumping 
necessary  all  vary  with  each  separate  case,  so  for  the  cost  and  rate  of 
sinking  of  gravel  shafts  no  rule  can  be  laid  down.  If  values  are  found 
to  be  uniform  in  a  number  of  preliminary  shafts,  the  subsequent  ones 
may  be  spaced  one  to  an  acre  or  one  to  each  5  or  10  acres,  according 
to  the  degree  of  uniformity  with  which  it  is  found  that  the  gold  is 
disseminated. 

Shaft  sinking  has  the  advantage  of  affording  large  samples,  gen- 
erally from  500  to  1,000  lb.,  giving  a  good  idea  of  the  amount  of  clay, 
size  of  boulders,  etc.,  and  the  character  of  the  bedrock.  Unfortunately, 
the  ground  easiest  for  prospect  by  shafts  is  generally  unfavorable  for 
working  on  account  of  lack  of  water,  presence  of  clay  or  heavy  depth 
of  soil  over-burden.  In  loose  gravel,  the  most  favorable  for  placer 
mining  by  any  form,  shaft  sinking  is  rarely  possible. 

Type  of  Drill. — The  engineer  is  frequently  unable  to  determine 
beforehand  what  method  of  testing  he  will  be  obliged  to  use.  In 
such  a  case,  it  is  essential  that  at  least  one  drill  should  be  sent  to  the 
property.  The  type  of  drill  will  be  determined  largely  by  conditions. 
As  regards  cost,  it  may  be  taken  as  an  average  that  the  heavy  power 
drill  with  10Q  ft.  of  six-inch  casing  will  cost  about  $3,000  (£600) 
landed  on  a  property  in  Alaska,  Siberia,  South  America,  or  Africa, 
while  the  ordinary  type  of  hand  drill  with  50  ft.  of  4-in.  casing  will 
cost  about  one-third  this  amount.  With  the  power  drill  it  is  absolutely 
essential  that  an  expert  machine  man  should  be  secured  in  advance, 
either  locally,  or  if  this  is  not  possible,  he  must  be  engaged  from  some 
field  where  placer  testing  by  power  drills  is  practiced.  It  is  not  ad- 
visable to  engage  an  oil  driller  or  well  driller  for  placer  drilling. 
Keystone  drillers  get  $3.50  per  shift  in  California,  and  from  $5  to  $6 


THE   SAMPLING   OF   PLACER   DEPOSITS  151 

and  board  in  Alaska.  The  average  expense  of  such  a  man  sent  to  an 
isolated  field  will  be  $7  per  day  plus  his  expenses  there  and  return. 
This  is  one  of  the  items  which  indicates  that  the  sampling  of  placer 
properties  is  not  a  cheap  process  in  any  country.  Firemen  and  even 
assistant  machine-men  for  operating  the  drill  on  night  shift  may  gen- 
erally be  locally  obtained. 

Hand  drills,  such  as  those  of  the  Banca  and  Empire  type,  require 
no  skilled  labor,  and  a  drill  crew  of  local  labor  can  be  taught  in  a  few 
hours  to  satisfactorily  operate  the  drill. 

Fanners  or  panmen  who  can  be  trusted  can  generally  be  obtained 
in  -placer  districts  such  as  California,  New  Zealand,  or  Alaska.  Great 
difficulty  is  experienced  in  getting  satisfactory  panners  in  some  regions 
and  this  adds  seriously  to  the  expense  of  examinations  in  those  coun- 
tries as  comparatively  highly  paid  assistants  must  be  taken  with  the 
expedition.  The  engineer  in  charge  will  always  do  as  much  of  the 
panning  of  samples  himself  as  possible,  but  on  examinations  of  any 
size  he  is  otherwise  employed. 

In  the  older  regions,  like  the  dredging  fields  of  New  Zealand  or 
California,  the  type  of  drill  in  local  use  will  generally  give  the  most 
satisfaction,  because  trained  drill  crews  may  be  engaged  before  the 
engineer  leaves  his  home  office  to  take  the  work  on  contract  at  stip- 
ulated rates.  Thus,  in  Nome,  Alaska,  drill  crews  operating  power 
drills  with  wide  tires,  especially  adapted  for  the  tundra,  will  contract 
for  drilling  in  frozen  ground  at  80c  per  foot,  and  in  unfrozen  ground 
for  $1.50  per  foot.  This  it  can  be  easily  seen  is  an  immense  advan- 
tage, as  the  engineer  is  not  obliged  to  purchase  an  expensive  drill  at 
a  distance,  have  it  shipped  to  Nome,  and  run  the  risk  of  losing  a  por- 
tion of  his  time  through  its  non-arrival. 

At  Oroville,  California,  or  in  the  Yuba  or  Folsom  fields,  drill  crews 
may  be  contracted,  at  so  much  per  foot  of  hole,  the  examining  engi- 
neer being  able  to  give  all  his  attention  to  the  washing  of  the  samples, 
watching  only  to  see  that  the  drilling  is  done  in  such  a  manner  that 
the  correctness  of  the  sample  is  not  interfered  with. 

In  isolated  parts  of  South  America,  Africa,  or  Siberia,  on  the 
other  hand,  where  such  contract  crews  for  gravel  work  do  not  ex- 
ist, and  where  roads  for  transport  are  generally  lacking,  it  will  gen- 
erally be  found  expedient  to  use  the  hand  type  of  drill,  if  possible. 
An  exception  is  in  the  deep  gravel  areas  of  the  Lena  and  Trans-Baikal 
regions  of  Siberia,  where,  notwithstanding  the  difficulties  of  transport, 
it  is  in  most  cases  advisable  to  use  the  power  drill. 

In  nearly  all  cases,  however,  of  an  extensive  examination  of  gravel 
deposits,  the  hand  drill  will  be  found  a  valuable  accessory.  No  ex- 
tensive gravel  examination  should  be  undertaken  without  such  a 
machine  as  a  part  of  the  equipment.  The  hand  drill  is  easy  to  trans- 
port, its  total  weight,  including  50  ft.  of  casing,  rods  and  necessary 
tools,  not  exceeding  2500  lb.,  while  the  heaviest  parts  weigh  less  than 
75  lb.  each.  It  is  quickly  assembled  in  the  field,  and  common  labor  in 
any  country  can  be  soon  taught  its  mechanical  features.  Such  drills 


152  MINE    SAMPLING   AND   VALUING 

have  been  successfully  used  not  only  in  English-speaking  countries, 
but  in  the  Malay  States,  South  America,  Africa,  Siberia,  and  else- 
where, using  native  labor.  I  have  seen  such  drills  transported  on  mule 
back  and  even  on  reindeer. 

Drilling  with  Power  Drills. — For  the  use  of  the  Keystone  or 
other  power  drill  the  ground  is  first  laid  out  in  squares  by  co- 
ordinates from  one  acre  to  in  some  cases  10  acres,  and  a  post  to  mark 
a  proposed  drill  hole  is  placed  at  each  corner.  The  drill,  which  is  of 
the  6-in.  type,  weighs  with  its  tools  about  eight  tons,  and  consists  of  a 
carriage  with  wide  wheels,  on  which  is  mounted  an  8-horsepower  steam 
engine  and  boiler  or  liquid  fuel  motor,  a  detachable  derrick,  a  walking 
beam  arrangement,  and  a  mechanism  for  adjusting  the  walking  beam. 
In  the  traction  type,  the  power  may  be  used  when  advisable  to  move 
the  drill  from  one  hole  to  another.  This  in  turn  gives  a  vertical  re- 
ciprocating motion  with  a  30-in.  stroke  to  a  1^4 -in.  cable  passing  over 
a  sheave  at  the  top  of  the  derrick.  The  rope  passes  down  the  front 
and  centre  of  the  derrick  frame  and  is  attached  by  means  of  a  rope- 
socket  and  a  heavy  4-in.  stem,  12  ft.  long,  to  a  long  thin  bladed  drill 
bit.  A  tool  called  the  jars  is  also  occasionally  inserted  between  the 
rope  socket  and  the  stem.  The  string  of  tools  so-called  weighs  from 
700  to  1,200  Ib.  according  to  the  number  and  size  used.  A  hole  is  dug 
with  pick  and  shovel  and  a  five-foot  length  of  casing  of  6-in.  inside 
diameter  pipe  fitted  at  its  lower  end  with  a  drive  shoe  of  which  the 
cutting  edge  is  7^?  in.  outside  diameter,  is  set  in  the  hole,  plumbed 
and  the  dirt  filled  in  around  it.  Next,  if  drilling  is  necessary,  the  drill 
bit  is  lowered  through  the  casing  and  a  few  inches  are  drilled  below 
the  casing.  If  the  ground  is  soft  a  driving  cup  is  screwed  to  the  pipe 
and  the  casing  is  driven  for  say  two  to  six  feet  through  the  top  soil 
or  sand.  This  description  of  driving  and  pulling  the  casing  is  quoted 
from  a  paper  by  N.  B.  Knox* 

"The  driving  is  accomplished  by  striking  the  driving  head  with  a 
couple  of  iron  blocks  clamped  to  the  stem  by  means  of  two  1^4-in. 
bolts,  the  weight  of  the  string  of  tools  acting  as  a  hammer.  After 
driving,  the  driving  blocks  are  removed  when  the  first  length  of  casing 
is  driven  down  to  head,  the  driving  cap  is  removed,  a  second  section 
of  casing  is  screwed  on  the  first,  the  driving  cap  replaced,  and  drilling 
resumed.  When  the  required  depth  is  reached,  determined  either  by 
striking  bedrock  or  passing  through  the  pay  stratum,  the  hole  may  be 
considered  finished,  and  the  next  step  is  to  pull  up  the  casing.  This  is 
accomplished  by  removing  the  bit,  stem  and  jars,  and  replacing  them 
by  what  is  known  as  the  pulling  or  pipe  jars.  These  consist  of  an 
iron  boss  fixed  to  the  end  of  a  rod  4*/2  ft.  long.  Above  the  boss  is  a 
1-in.  thick  plate — the  'knocking  head' — provided  with  threads  which 
are  screwed  into  the  sleeve  of  the  top  section  of  casing.  The  stem  of 
the  boss  passes  through  a  square  hole  in  the  plate.  The  walking  beam 
is  set  into  motion,  and  the  string  of  casing  is  raised  by  the  boss  strik- 
ing against  the  knocking  head.  As  each  section  of  casing  is  raised, 

*Trans.    Inst.    Min.    and    Met.,    Vol.     12;     1902. 


THE    SAMPLING    OF    PLACER    DEPOSITS 


153 


r 

*Jr^—4 

1       „': 

1       x 

1 
"^\l 

] 

| 
i 

\ 

1 

1 

1 

!      § 

n  1 

^  I 

i  1  i= 

Bi 

1 
I 

ih 

t 

H 

1 

i    / 

\     ' 

1 

1 

i 

1 
i 

/  — 

l""^ 

-r___ 

___i 

Thi 
Ptecer  8,/- 
*%"  cuH.'n    bladt. 


V 


CASING    AND    DRILLING 

BIT    FOR    6"    POWER 

DRILL   WITH    DRIVEN 

CASING. 


Fig.    23. 

KEYSTONE    8    FT.    X 
IN.    SAND-PUMP. 


4"  HAND   DRILL  WITH 

ROTARY  CASING,  SHOW 

ING    SAND-PUMP    AND 

DRILL  ROD. 


154  MINE    SAMPLING   AND   VALUING 

it  is  unscrewed,  and  the  knocking  plate  screwed  on  the  next.  If  care 
is  used  in  keeping  the  threads  of  the  casing  clean,  the  casings  can  be 
used  for  a  long  time.  It  is  rarely  that  a  casing  is  lost." 

Great  care  is  necessary  that  the  drill  bit  is  never  allowed  to  go 
below  the  casing,  but  from  12  to  24  inches  of  the  core  must  be  allowed 
to  remain  in  the  pipe.  The  sand-pump,  which  is  attached  to  a  separate 
small  line  and  operated  by  a  separate  reel,  is  lowered  into  the  hole 
every  foot  drilled,  and  the  drillings  pumped  out  within  6  inches  of  the 
bottom  of  the  core,  the  pump  is  lifted  and  its  contents  dumped  into  a 
box  about  8  feet  long  and  12  in.  wide.  The  depth  of  core  is  carefully 
measured  each  time  by  a  system  of  marking  the  drill  stem  and  compar- 
ing the  relative  depths  below  surface  of  the  top  of  the  core  and  the 
length  of  the  casing. 

Each  foot  of  the  core  is  panned  and  the  slimes  are  afterwards 
treated  in  the  rocker.  The  gold  from  each  panning  is  estimated  and 
classified  as  to  number  and  size  of  colors,  and  the  appropriate  entry 
made  in  the  drill  log  (Fig.  24).  After  bedrock  is  reached  and  the  hole 
finished,  the  slime  from  all  the  pannings  is  rocked  and  the  fine  gold 
recovered  is  added  to  the  samples.  All  the  gold  is  then  amalgamated 
with  a  globule  of  quicksilver,  the  amalgam  placed  in  a  bottle  and  after- 
ward freed  with  acid  from  the  quicksilver  and  weighed.  The  details  of 
entries  in  the  drill  log  and  the  time  log  (Fig.  25)  shown  here,  which  are 
for  hand  drill  work,  indicate  the  depth  of  hole,  character  of  material, 
bedrock,  whether  the  drill  bit  gets  below  the  casing,  so  as  to  endanger 
salting  the  result,  but  also  an  important  factor,  at  what  depth  the  pay- 
layers  are  found,  and  the  horizons,  at  which  the  gold  is  concentrated. 
From  one  to  two  feet  are  drilled  in  bedrock  even  after  no  more  colors 
are  found,  for  the  sake  of  safety.  Water  is  always  kept  in  the  hole 
artificially  if  it  does  not  flow  into  the  hole  naturally  from  the  ground. 

Good  Keystone  work  runs  from  20  to  30  ft.  of  drilling  in  12  hours, 
and  the  cost  averages  in  standard  California  practice,  including  the 
services  of  the  panner,  $2  per  foot  drilled.  In  Alaska  in  the  summer 
of  1912,  in  solidly  frozen  ground,  where  casing  was  almost  entirely 
dispensed  with,  2,500  ft.  of  drilling  was  done  with  a  heavy  specially 
designed  power  drill  with  6%-in.  bit,  at  a  cost  of  approximately  $1.15 
per  foot.  About  40  ft.  per  shift  was  made,  with  a  fuel  consumption 
of  about  half  a  cord  of  wood,  and  1,000  gallons  of  water  for  drilling 
and  washing  samples. 

Calculation  of  Value. — The  value  of  the  ground  is  calculated  as 
follows:  The  cylinder  of  material  represented  by  the  core  is  7*/2  in. 
in  diameter  multiplied  by  the  depth,  if  the  drilling  has  been  so  careful- 
ly done  as  to  allow  no  gravel  to  run  into  the  pipe.  Each  foot  of  the 
core  contains  0.3  cu.  ft.  or  1/30  of  a  cu.  yd.  In  the  case  of  a  hole 
40  ft.  deep  from  which  12c.  in  actual  gold  was  recovered  there  would 

40                                             12  X  90       070 
be-Q^cu.  yd.  worth  12c.,  or    — —  =27.2c.  per  cu.  yd.     In  this 

case  the  factor  used  is  0.3.     It  has  been  found  by  elaborate  experi- 
ments that  this   factor  is   somewhat  too  large,   and  0.27  is  the  one 


THE   SAMPLING   OF   PLACER   DEPOSITS 


155 


DRILL    LOG 


£,    Crew  Ho. 


Sfarfed  Hole 


Dafe 

.  .................  Trac/ 

Finished  ................ 


.  .....  1 


Depths 
rxr/v/>  /A/. 


h 

se 


Depth  d 
below  p 


Core  fo 
dopth«d 


Core 
pum 


Formation 


»».a 


Qtot, 


\o 


•  o 


dLo 


\s 


Ao. 


/o 


11 


13 


iul 


12 


13 


15. 


/S 


12 


Q 


M 


tl 


IS 


-fc 


do 


19 


1! 


20-4 


d 


To/a/  Depf/j     *XJ 

Depth  to  Bedrock    17  jf, 

Depth  to  Water  Level  J  X/f  ...££. 

Measured  Volume     I  <  31  CM*  ^ 


Panman 


,  Fig.   24.     DRILL   LOG. 


156 


MINE    SAMPLING   AND   VALUING 


TIME    LOG 


Hole  No.  foJluuL  £       Crew  No. J Tract    I 

Started  Motel^»3X^>»...lu£*|.J.r.ia.  Finished IL S 'QQ- 


fill" 


Nature 

of 
work 


Hr.     Min.          Consumed  in 


rilling 
peratio 


3 

O  Su  a 


Dept 
pipe 


p./vy 


1* 


ar 


<& 


.ca^rtOirT 

iS 


»o 


3.00 


\S 


/o 


3-O. 


3.13 


3. 


If 


ol- 


^.oo 


w 


ULM  tun) 


/7 


W 


11.  IS 


ij.ro 


Pan/nan 


Fig.  25.     TIME  LOG. 


THE   SAMPLING   OF   PLACER   DEPOSITS  157 

ordinarily  used.  This  means  that  a  cylinder  100  ft.  long  equals  1 
cu.  yd.  In  the  illustration  just  given,  the  result  would  be  30c.  per 
cu.  yd.,  using  0.27  as  a  factor. 

Formula. 

Let        C  =  money  value  of  sample 
D  =  depth  in  feet 
V  =  value  per  cubic  yard 
.  L  =  length  of  cylinder  which  will  equal  1   cubic  yard 

Then     C  X   L_ 
D 

or  substituting  the  figures  above  quoted  we  have: 

12  X  100 
-40-      =30c' 

Looking  now  at  the  'Prospecting  Report  Sheet'  (Fig.  26)  on 
which  the  final  results  for  each  hole  are  calculated,  the  data  necessary 
in  calculating  the  final  results  are  shown  under  the  heads  'Average 
value  per  cubic  yard'  and  'Depth  used  in  calculation.'  These  two 
multiplied  together  give  .the  number  of  foot-cents  for  each  hole. 
The  foot-cents  added  together  and  divided  by  the  number  of  feet 
drilled  give  the  average  value  per  cubic  yard  for  the  gravel  of  a 
given  line  of  holes,  or  a  given  block  of  ground,  providing  the  holes 
are  equally  spaced,  while  the  average  depth  in  feet  is  the  total  footage 
divided  by  the  number  of  holes.  Erratic  results  on  the  high  side  are 
neglected,  and  where  the  values  gradually  decrease  on  both  sides 
of  a  large  tract,  the  engineer  will  finally  draw  lines  on  the  completed 
drilling  plan,  which  define  an  area  of  pay  ground.  If  this  pay- 
channel  has  sufficient  width  and  length  to  afford  a  yardage  for  a 
reasonable  life  for  the  extent  of  equipment  in  dredges  or  other  ap- 
paratus which  is  practicable,  and  yield  the  desired  margin  of  profit, 
the  property  may  be  recommended. 

Drilling  with  Hand  Drills. — Where  depths  of  material  from  sur- 
face to  bedrock  do  not  exceed  40  ft.,  the  hand  drill  may  be  used. 
There  are  three  types  of  these  drills  in  common  use,  two  which 
are  practically  of  the  same  design  and  known  respectively  as  the 
Banca  and  the  Empire,  and  a  third,  which  differs  in  design  and  mode 
of  operation,  is  known  as  the  Dimond.  They  all  accomplish  prac- 
tically the  same  result,  in  the  same  time,  and  at  the  same  cost. 

In  isolated  localities,  devoid  of  roads,  and  where  the  time  factor 
is  important,  the  hand  drill  has  many  things  in  its  favor.  I  do  not 
recommend  its  use  where  it  can  be  avoided,  for  the  following  reasons : 

1.  Instead   of   six-inch   casing,   only    four-inch   is   used,   whereby 
a  smaller  sample  is  obtained. 

2.  It  has  not  the  power  to  penetrate  heavily  frozen  ground  or 
large  boulders. 

3.  The  type  of  sand-pump  used  is  of  the  ball  type  rather  than  the 
clap  valve,  and  the  ground-up  material  brought  to  the  surface  is  not 
as  certain  to  represent  the  exact  portion  of  the  hole  as  in  the  case 
of  the  power  drill. 


158 


MINE    SAMPLING   AND   VALUING 


4.  There  is  a  greater  opportunity  for  tampering  with  the  sample,  as 
four  men  stand  continually  on  the  platform  in  drilling,  and  although 
there  is  greater  danger  from  salting  in  shaft-sinking,  this  danger  does 
not  exist  to  any  extent  in  the  use  of  power  drills. 


PROSPECTING  REPORT 


Date 


i  -mi- 


No,  on  Map 

>K9  S .    OVi/ 


juu^a 

Fished 


uuJJL    I     to     .5- 

Machine  Used 

T-  MY 

</      <7 


Afo.  o/  Hole  (fiii+uL.    E 

Location  of  Hole    f  CO 

Method  of  Prospecting  by    IJ-CUXxV 

SfarU  Wo/e  J2-  '  3 4*  ^.    >T7 .  J^^/    '//  2- 

Dcp/ft  o/  Top  Soil  ...—. 

Dcp(/i  lo  Bedrock-     II   &t~ -- -    

Depth  Drilled  - 2-1    JT*-^  • - 

Depth  Used  in  Calculations     3.O 

Depth  lo   Water  Level /   '/I/. 

Distribution  of  Cold      IO,.ll,     IT-  ,    /4  ,     H ,    18  ,     II, 

Nature  of  Top  Soil   ...>LO     %#f\     4L*rL£. 

Nature  of  Gravel  c^rvrit^    A^    ^T^J^cLtju^rrt     -Xx-aA-^", 

Nature  of  Cold  Bearing  Strata  t7>U/CjC     OXl  O^-cC  QLXX^.   A&AJ-4 ,    C. 

Nature  of  Bedrock  S  Stfrf  $O    >TUJu^Uy*VO  A  «xu)"-^i  O^tuu^Ji.3 

Time  Consumed  in— Moving ^-P...-.>*1.- .Setting  Up     .    3.  J~  >T? ..  ... 

Pulling  Pipe  2.«f~    >T1  , Delays .....  Total 

Causes  of  Delays.^ 


Cre 


OM- 


. 
d    XUx/T 


Drilling  Operation!. 


..  2J~ 


7  .-WVi 


Mg.  o/  Cold  Recovered Jj^. 

Factor*  Adopted 

Cu.  F 1.  per  Fl.  Depth  of  Hole.-  _,/7  3  ^ 
Remarks    _ 


of  Cold  .  Ot,  .4- 

Average  Value  per  Cubic  Yard    ~J  ,    %  S  7  4- 


Signed          fAAs  WftuML,  .., 

Checked  by  ..........................  ._._  .......................  ________________________  ..........  .  ..............  .  ............. 


Approved  by 


Fig.    26.     PROSPECTING    REPORT. 


The  operation  of  the  hand  4-in.  drill  with  rotated  casing  is  as 
follows :  The  hole  is  started  with  an  auger  or,  if  necessary,  by  digging 
for  18  inches.  In  the  case  of  soft  peat  or  soil  over-burden  several  feet 
may  be  made  with  the  auger. 


THE    SAMPLING   OF   PLACER   DEPOSITS  159 

A  5-ft.  length  of  4-in.  casing  with  a  0.38-ft.  diameter  cutting  shoe 
screwed  to  the  bottom  is  set  in  the  auger  hole  and  plumbed,  and  the 
heavy  platform  is  screwed  on  top  and  four  men  mount  on  it.  The 
men,  platform  and  casing  will  weigh  from  1,000  to  1,500  lb.,  accord- 
ing to  the  depth  attained  and  amount  of  casing  used.  A  heavy  log 
is  now  used  as  a  ram  to  drive  the  casing  for  two  or  three  feet  into 
the  ground.  For  medium  ground  this  driving  process  is  not  much 
employed  after  starting  the  hole.  It  is  made  use  of  in  tight  or 
coarse  gravel,  in  which  case  also  a  drilling  bit  is  used  with  the  string 
of  rods  as  well  as  the  drilling  pump.  The  ordinary  procedure  is  to 
rotate  the  pipe  by  means  of  long  rods  or  a  sweep  propelled  in  a  circle 
by  a  horse,  the  weighted  platform  causing  the  casing  to  sink.  A  string 
of  rods,  graduated  in  inches  for  measurement,  and  with  the  drilling 
pump  screwed  to  the  bottom,  is  let  down  the  hole,  and  drilling,  pump- 
ing, and  rotating  go  on  all  at  the  same  time.  This  combined  work 
is  an  advantage  of  this  type  of  drill,  not  possible  with  the  power  drill. 
The  same  care  is  taken  as  in  Keystone  drilling  never  to  let  the  pump 
or  bit  get  below  the  casing,  and  there  is  no  essential  difference  in 
the  mariner  of  treating  the  material  as  it  is  drawn  up  each  time  by 
the  pump,  dumped  into  the  clean-up  box  and  panned,  except  that 
smaller  amounts  are  dealt  with. 

When  the  casing  is  driven  to  bedrock  and  the  hole  finished,  it  is 
pulled  by  means  of  a  clamp  and  long  lever  acting  on  a  frame  bolted 
together  of  three  pieces  of  structural  steel. 

The  system  of  keeping  the  notes  is  practically  the  same  as  with 
the  power  drill,  and  the  sheets  here  exhibited  (Figs.  24,  25,  26)  are 
in  fact  records  of  Empire  hand-drill  work.  The  rate  of  progress  per 
day  varies  considerably.  It  is  safe  to  put  it  at  15  to  20  ft.  per  10-hour 
day  counting  all  time  consumed.  The  cost  of  drilling,  including  salary 
of  the  assistant  or  panner  in  charge,  will  range  from  50c.  to  $1.15 
per  foot.  Counting  labor  alone  it  will  be  from  25c.  to  40c.  per  foot. 
A  chisel  bit  is  used  to  cut  through  small  boulders,  but  much  time  is 
lost  when  large  boulders  are  encountered,  for  it  is  useless  to  try  to 
drill  through  them,  and  if  they  exist  in  large  numbers,  hand  drilling 
is  generally  labor  lost  ;  otherwise  the  drill  will  successfully  cope  with 
a  surprising  number  of  difficulties. 

In  calculating  the  value  of  the  gravel  instead  of  0.27  the  pipe- 
factor  used  is  0.1134.  This  is  the  volume  of  1  foot  in  length  of  core, 
which  is  a  cylinder  0.38  ft.  in  diameter,  the  size  of  the  cutting  shoe. 

It  takes  238.1  ft.  of  core  to  make  1  cu.  yd.  instead  of  100  ft.,  as 
with  the  6-in.  power  drill.  Thus  if  the  total  gold  found  in  the  hole 
amounts  to  6c.  and  the  depth  30  ft.,  then  applying  the  formula  we 
have: 


or   substituting  -  '-=:47.6c.   per  cu.   yd. 

o  \) 

When  the  value  has  been  determined  the  averaging  of  the  holes 
is  figured  as  previously  described. 


160  MINE    SAMPLING    AND   VALUING 

•"•A:  few  test  shafts  should  be  sunk,  if  possible,  around  the  drill 
holes  which  have  been  finished,  to  test  the  accuracy  of  the  results. 
Unfortunately,  this  is  seldom  possible  on  account  of  water.  It  may 
be  said,  however,  that  the  drilling  of  placer  gravels,  both  with  hand 
and  power  drills,  has  long  passed  the  experimental  stage.  With  the 
allowance  of  the  safety  factor,  which  every  engineer  will  make  for 
difficulties  attending  recovery  of  the  prospected  values  in  the  subse- 
quent mining,  it  is  doubtful  if  drilling  can  be  superseded  by  any 
system  of  prospecting  alluvial  gravels  which  is  more  rapid,  economical 
or  generally  satisfactory. 


INDEX 


A  Page. 

Alluvial   deposits,   classes   of 142 

Amortization  of  capital    116 

Theory  of 118 

Assay  plans    91 

Assaying,    preparation    for 69 

Of  samples  77 

Assays,  high  48 

Erratic  high    88,  89 

Averages,  calculation  of 83,  94 

Arithmetic    86,  96 

Gravimetric    96 

Volumetric  86,  95 

From  drill  holes   100 

Average  value,  along  a  working.     93 
Value,  in  drilling  placers  .... 

154,   157,   159 

Width  and  value 94 

B 

Base    metal   prices 112 

Blocks     . . 

Cubical    contents   of    94 

Mine  dividend  into    93,  110 

Opened  on  two  sides 107 

Broken   Hill   South   Blocks  mine 

46,  107 

Hill     South     mine,     probable 

ore    in    107 

Brunton,   D.  W.  quoted 71,  74 

Bucking  board 75 

Bucket,    duck   sampling    21 

Buell,  L.  T.,  acknowledgment  to 

7,  58 

Drilling   at    Miami    . . .  , 60 

Burma,   silver-lead   mine  in 69 


Calculation   of   average   value...  87 

Of   tonnage    93 

Of  true  width   sampled.. 42 

Calculations  of  value,  in  drilling 

placers    154,  157,  159 

Camp   Bird  mine    . . . : 53 

Channels    in   gravel    deposits....  147 

Checking    surveys    132 

Check   sampling    88 


Chisels 20 

Churn    drilling    .  . . . . . . . ;  ;v: '.  ,- 

As  applied  in  sampling......  57 

At    Miami    60 

At  Ray  Central    60 

At   Giroux ....,...*.  62 

Errors    in 59 

Log    for 61 

Clearing   rock    exposures 29 

Clean-up  boxes  for  placer  work.  149 

Cross-cuts,    sampling    of 41 

Crucibles   for  assaying   . . . ... . . .  77 

Crushers    for   sampling    :..;.-.'.:  71 

Crushing  of  samples    . ; : . . : : . . .  70 

Fineness   of    . .  . .  74 

With    hammers 73 

With   bucking   board    ......;  75 

Cupellation 79 


Description  of  sample  65 

Diamond  drill,  compared  with 

churn  drill 57 

Disseminated  copper  deposits. v,  57 

Dividers,  mechanical 25 

Division  of  sample  72 

Dressing  mines  for  sale 133 

Drill  holes,  calculations  from...  100 

Drill  log  for  placer  work 155 

Drilling  placers,  calculation  of 

average  value  .154,  157,  159 

Drilling  with  power  drills 152 

Drills  for  testing  placer  ground,  150 

Drying  mine  samples  r  70 

Dust  in  samples 67 


E 


76 


Envelopes   for   pulp   samples 

Equipment    for    ........ 

Examining    placers    148 

For  mine  sampling   19 

Errors  in  churn-drill  sampling..     59 
Estimation    of   ore 

Of  'ore   in  sight'    

Examination    of    prospects 

Of  old   mines    . 


83 
104 
129 
130 


Exposure,  preparation  of    ......     29 


162 


MINE    SAMPLING    AND   VALUING 


F  Page. 

Fineness  of  pulp    75 

Finlay,  J.  R.,  price  of  copper...  114 

G 

Gads  20 

Garthwaite,  E.  H.,  quoted 88 

Geological  factors  50 

Problems  53 

Giroux  mine,  churn-drilling  at. .  62 
Goldfield,  Nevada,  monograph 

on  54 

Gravel  deposits,  classes  of 142 

Deposits,  origin  of  146 

Gravimeti'ic  averages  96 

H 

Hammers  for  sampling 19 

Crushing    with     73 

Hand  drills  for  testing  placers..  157 

Handling  of  samples    64 

High    assays    48 

Assays,   erratic    88 

Assays,  discussion  of   89 

Hoover,   H.   C,   acknowledgment 

to 7,  87 

Definition  of  'ore  in  sight'. . .  105 

'Principles  of   Mining'    ..108,  113 

Theory  of  amortization   118 


Igneous  rocks,  table  of 55 

Implements,   for  mine   sampling.  19 

Inaccessible  ore    99 

Inclined  orebodies,   sampling  of.  34 
Instruments    and    equipment    for 

examining  placers    148 

Interval    sampling 31 

Irregular  roof,  sampling  of 37 


Jones    sampler    26 

K 

Kemp,  J.  F.,  definition  of  ore...  12 

Keystone  drill   58 

Power   drills    152 

Knox,   N.   B.,   quoted    152 

Krumb,     H.     R.,     sampling     ap- 
paratus      63 


Labels,  for  samples 24 

Lawrence,    B.    B.,    definition    of 

'ore  in  sight'    104 

Log  for  churn  drilling 61 

For  drilling   placers    155 


M  page. 

McNeill,       Bedford,       table       of 
world's  production  of  metals, 

quoted    114,  119 

Measurement  of  width  sampled.  43 

Methods  of  sampling   28 

Miami   mine,   drilling   at 60 

Minerals,   specific  gravity  of 138 

Misrepresentation   of   facts 134 

Moils    20 

Munroe,   H.   S.,  acknowledgment 

to    7 

Quoted   91,  95,  97 

N 

Numbering  the   sample    64 


Ore,  definition  of  12 

Estimation   of    83 

Inaccessible     99 

In    sight,    definitions 103 

Positive,     probable     and    pos- 
sible      104 

Proved     105 

Prospective     105 

Reserve  plans   109 

Reserves,    tonnage   of 109 

Outcrops      52 


Pay  channels  in  placers   147 

Pick    analysis    16 

Pick,  prospector's 21 

Placers,  buried    143 

Creek,  hillside,  beach 144 

Lake-bed  and  harbor    146 

Origin   of    146 

Pay  channels  in    147 

River  bar,  seabeach   145 

Plate's  sampling  device   63 

Porphyry  copper  deposits    57 

Positive    ore    104 

Possible  ore   104 

Precautions   in   sampling    45 

Preparation      of     exposure      for 

sampling    29 

Of  samples  for  assay.... 69 

Prices  of  base  metals 112 

Principles    of    sampling 11 

Probable  ore    104,  105 

Probert,  F.   H.,  acknowledgment 

to     7 

Drilling    58 

At  Miami   60 

Profits,    calculation    of Ill 

Prospecting  report,  drilling  plac- 
er ground    158 

Prospective  ore   105 

Prospects,    examination    of 129 


INDEX 


163 


Page. 

Proved  ore  105 

Psychology  and  mine  valuing...  11 
Pulp,  preparation  of  75 

Fineness  of  75 

Purington,  C.  W 

Sampling  of  placer  deposits..   140 


Quantity   of   ore   per    foot    sam- 
pled         30 

Quartering  samples   73 

R 

Rand  banket,  uniformity  of...  108 
Ray  Central  mine,  churn-drill- 
ing      60 

Record  book  for  sampling  ....  84 

Re-opened  old  mines    129 

Reports,   writing  of    120 

Re-samples     66 

Rickard,    T.     A.,     acknowledg- 
ment to    7 

Definition   of   ore    14 

'Sampling  and   Examination 

of  Ore' 28,  30,  71,  87,  104 

Thantom  Profits'    112 

Rise,  sampling  at  a    38 

Sampling  of    39 

Rocker  for  placer  work   149 

Rocks,    classification    of 51 

Igneous     55 


Sack,  for  carrying  samples 23 

Sacks,   canvas   sample    24 

Sacking  the  sample    67 

Salting  of  mines    125 

Sample   envelopes    76 

Samplers,    for   churn    drills 62 

Jones    26 

Miami 58 

Plate's   device    63 

Sampling  at  a  rise 38 

By  churn   drilling    57 

Calculations   157,  159 

Check 88 

Crooked   workings    32 

Cross  cuts   41 

Daily  mine 15,  16 

Fractional 33 

General  methods  28 

Interval     31 


Page. 

Irregular   roof    37 

Of  placer  deposits,  general..  140 

Precautions    45 

Record    book    84 

Rises  and  winzes   39 

Underground    28 

With   drills    150 

With    shafts    149 

Seals     24 

Sealing  sample  sacks  67 

Wax   24 

Shaft-sinking     for     testing 

placers     149 

Silver  Islet,  lode  formation  at.  53 

Spacing  of  drill  holes  100 

Special  cases  132 

Specific  gravity,  definition  of..  137 

Determination    of    137 

Of  minerals  138 

In   calculating  averages....  96 

Star   drill    58 

Stretch,  R.  H.,  definition  of  ore  12 

Surveys,  checking  of   132 


Tapes,   for  measuring    21 

Tempering   moils    21 

Time  log  for  drilling   156 

Tonnage,   calculation  of   93 

In  a  block  of  ore 95 

Of  ore  reserves   109 

Treatment   problems    Ill 

Tying  and  sealing  samples....  67 

V 

Value  of  placer  ground,  calcu- 
lations     157,    159 

Vendor's    reports    12 

Volumetric  method  of  averag- 
ing assays    95 

W 

Waste,  in  tonnage  calculations,  110 

Way,  E.  J.  quoted    86 

Weight  of  samples  30 

Whitewash  for  marking  walls...     28 
Wide  orebodies,  sampling  of,  33,  38 

Width,  measurement  of  43 

Winze,  sampling  of  39 

Working  costs    Ill 

World's  production  of  metals.    114 


DM   ... 


oar... 

'II       91G 

V£f     tj[  .V,    , 

?-; 

IS      ,.,...,.    ^nb 

IS 


UNIV 


JFORNIA  LIBRARY 
BERKELEY 


Return  to  desk  from  which  borrowed. 
This  book  is  DUE  on  the  last  date  stamped  below. 


8 


1956  *-tt 


TMar'57BR 


JUN4-   1957    Si 

C'D  OS 

MAY  31  1957 


LD  21-' 


OH 

,'48(B399sl6)476 


LTD 


p 


I    /  I 


291134 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


I 


